I always wondered this

[Anime is life]

Since sr3 gat hasn't had his neck tattoos what happened to them some people used to say they were covered by the collar but we can tell now they are gone but see he couldn't have got them removed because shaundi still has the scar from where she got her tramp stamp removed and he has none

 

[Singa]

The Zin skinned him and ate his skin along with the tattoos in order to gain his invincibility.

 

[Ruruko]

Most canonical approach I would say, is that the alien goo probably burned it off or reacted to it some way or something.

This is about as plausible as the post above me, though. I have no idea how right I am.