I just went gay all of a sudden!

No. The line has nothing to do with homosexuality.

That's pretty gay.

Discovered homosexualism?
It always existed - no one "discovered" it.

And yes, they know what it meant. But "gay" in those times just meant happy.

"You're not a real actor in this business until you've played a bitch. Or a psychopath killer."

"They already discovered homosexualism back in 1938?"

Yeah, I think they found homosexuality around 1929. It was hiding in the closet.

They already discovered homosexualism back in 1938?

From what I understand the word "gay" was used meaning "homosexual", but was yet to be a popular term. Again, from what I heard, the use in the film was a joke that was not meant to be understood by all.

I won't fight on this, as I'm not sure, but it is what I've understood.

From Wikipedia:

The term gay was originally used, until well into the mid-20th century, primarily to refer to feelings of being "carefree", "happy", or "bright and showy"; it had also come to acquire some connotations of "immorality" as early as 1637.

The term later began to be used in reference to homosexuality, in particular, from the early 20th century, a usage that may have dated prior to the 19th century. In modern English, gay has come to be used as an adjective, and occasionally as a noun, that refers to the people, practices, and culture associated with homosexuality. By the end of the 20th century the word gay was recommended by major style guides to describe people attracted to members of the same sex.At about the same time, a new, pejorative use became prevalent in some parts of the world. In the UK, U.S., and Australia, this connotation, among younger generations of speakers, has a derisive meaning equivalent to rubbish or stupid (as in "That's so gay."). In this use the word does not mean "homosexual", so that it can be used, for example, of an inanimate object or abstract concept of which one disapproves, but the extent to which it still retains connotations of homosexuality has been debated.

.... It was apparently not until the 20th century that the word began to be used to mean specifically "homosexual", although it had earlier acquired sexual connotations.

Sexualization
The word had started to acquire associations of immorality by 1637 and was used in the late 17th century with the meaning "addicted to pleasures and dissipations." This was by extension from the primary meaning of "carefree": implying "uninhibited by moral constraints." A gay woman was a prostitute, a gay man a womanizer and a gay house a brothel.

The use of gay to mean "homosexual" was in origin merely an extension of the word's sexualised connotation of "carefree and uninhibited", which implied a willingness to disregard conventional or respectable sexual mores. Such usage is documented as early as the 1920s, and there is evidence for it before the 20th century, although it was initially more commonly used to imply heterosexually unconstrained lifestyles, as in the once-common phrase "gay Lothario", or in the title of the book and film The Gay Falcon (1941), which concerns a womanizing detective whose first name is "Gay." Well into the mid 20th century a middle-aged bachelor could be described as "gay", indicating that he was unattached and therefore free, without any implication of homosexuality. This usage could apply to women too. The British comic strip Jane was first published in the 1930s and described the adventures of Jane Gay. Far from implying homosexuality, it referred to her free-wheeling lifestyle with plenty of boyfriends (while also punning on Lady Jane Grey).

A passage from Gertrude Stein's Miss Furr & Miss Skeene (1922) is possibly the first traceable published use of the word to refer to a homosexual relationship. According to Linda Wagner-Martin (Favored Strangers: Gertrude Stein and her Family (1995)) the portrait, "featured the sly repetition of the word gay, used with sexual intent for one of the first times in linguistic history," and Edmund Wilson (1951, quoted by James Mellow in Charmed Circle (1974)) agreed. For example: "They were ...gay, they learned little things that are things in being gay, ... they were quite regularly gay.

—Gertrude Stein, 1922

The 1929 musical Bitter Sweet by Noel Coward contains another use of the word in a context that strongly implies homosexuality. In the song "Green Carnation", four overdressed, 1890s dandies sing:

"Pretty boys, witty boys,
You may sneer
At our disintegration.
Haughty boys, naughty boys,
Dear, dear, dear!
Swooning with affectation...
And as we are the reason
For the "Nineties" being gay,
We all wear a green carnation."

—Noel Coward, 1929 , Bitter Sweet

The song title alludes to Oscar Wilde, who famously wore a green carnation, and whose homosexuality was well known. However, the phrase "gay nineties" was already well-established as an epithet for the decade (a film entitled The Gay Nineties; or, The Unfaithful Husband was released in the same year). The song also drew on familiar satires on Wilde and Aestheticism dating back to Gilbert and Sullivan's Patience (1881). Because of its continuation of these public usages and conventions – in a mainstream musical – the precise connotations of the word in this context remain ambiguous. Through the mid 20th century, the term "gay" commonly referred to "carefree", as illustrated in the Astaire and Rogers film The Gay Divorcee.

Other usages at this date involve some of the same ambiguity as Coward's lyrics. Bringing Up Baby (1938) was the first film to use the word gay in apparent reference to homosexuality. In a scene where Cary Grant's clothes have been sent to the cleaners, he must wear a lady's feathery robe. When another character inquires about his clothes, he responds "Because I just went gay...all of a sudden!" However, since this was a mainstream film at a time when the use of the word to refer to homosexuality would still be unfamiliar to most film-goers, the line can also be interpreted to mean "I just decided to do something frivolous." There is much debate about what Grant meant with the ad-lib (the line was not in the script). The word continued to be used with the dominant meaning of "carefree", as evidenced by the title of The Gay Divorcee (1934), a musical film about a heterosexual couple. It was originally to be called "The Gay Divorce" after the play on which it was based, but the Hays Office determined that while a divorcee may be gay, it would be unseemly to allow a divorce to appear so.

Shift to "homosexual"

By the mid-20th century, "gay" was well-established as an antonym for "straight" (which had connotations of respectability), and to refer to the lifestyles of unmarried and/or unattached people. Other connotations of frivolousness and showiness in dress ("gay attire") led to association with camp and effeminacy. This association no doubt helped the gradual narrowing in scope of the term towards its current dominant meaning, which was at first confined to subcultures. Gay was the preferred term since other terms, such as "queer", were felt to be derogatory. "Homosexual" is perceived as excessively clinical, since the sexual orientation now commonly referred to as "homosexuality" was at that time a mental illness diagnosis in the Diagnostic and Statistical Manual of Mental Disorders (DSM).

In mid-20th century Britain, where male homosexuality was illegal until the Sexual Offences Act 1967, to openly identify someone as homosexual was considered very offensive and an accusation of serious criminal activity. Additionally, none of the words describing any aspect of homosexuality were considered suitable for polite society. Consequently, a number of euphemisms were used to hint at suspected homosexuality. Examples include "sporty" girls and "artistic" boys, all with the stress deliberately on the otherwise completely innocent adjective.