So as long as you have a bit of zombie gut on your face...

If this was the case, then how come Rick et al aren't using it more extensively in their excursions?

Only in FTWD can a bit of blood on your face work like that. Applying it in direct view of the zombies and talking loudly at the same time also works. On TWD, characters need large amounts of guts all over them, to move very slowly, and to be absolutely silent for it to work.

As for why Rick's group don't do it more often, because it'd largely be pointless if sh!t ends up hitting the fan. Whether it's a helicopter about to fall through a roof, trying to save a team member impaled on metal things, attacked by a lurking zombie underwater attracted by one's movement... the guts trick would be rendered useless in high-stress situations where talking and quick movement is essential, or they don't know there is an unseen threat until it's too late. The guts trick is a last resort when cornered by a huge crowd and the only way out is to sneak past them.

When things go wrong on their supply runs, it's usually not something that could have been handled by the guts trick.

A piece of the world is still missing.

Because there is a special method to use this trick : hopefully you have to keep your mouth shut (meaning NOT having a stupid whinning kid in the bunch, like in TWD), or not be afraid of the dead (they're slow and easy to control, seriously and people from the Colonia were definitely not afraid of them), or being a natural tongue-tied guy (like Nick) who's more likely a zombie himself, and ultimately not walking too fast,....you must smell like the dead (so if you smell too good that can't work),

You see, too many criteria to be satisfied...lol

I dunno maybe in fear you don't need much blood since the zombies are still kind of fresh. But in walking dead, the zombies are more decayed so might need more blood to hide amongst them

its not gut , they just smear some blood on their face lol