Gaten was on set Thursday in Georgia, chatting it up -- seemingly about his scene -- in between takes ... wearing the iconic Hellfire Club shirt in front of a grave for Eddie Munson, the loveable misfit played by Joseph Quinn who died last season.

Eddie and Gaten's character Dustin Henderson were thick as thieves in the show, BTW ... so you can bet this will be a pretty emotional scene -- and on another note, GM's grown his hair out too ... very Eddie-like.

Now, addressing the elephant in the room ... Gaten doesn't look like the kid he was when this whole thing premiered in 2016 ... the dude's 21 now, so keeping him as a teenager on the show is tricky -- and yet, that's exactly what they're trying to sell here.

We should note ... the entire cast looks much older than their character's ages -- and fans have been pointing that out for a while ... but it's unclear if this final season will involve a time jump of some sort, or will pick up where season 4 left off. Time jump would help make this go down easier in terms of buying the characters' matured appearances.

BTW, Gaten's made a huge name for himself following his 'ST' breakout -- he was in the movie "Honor Society," appeared on Broadway in shows like "Dear Evan Hansen" and "Sweeny Todd," and hosted/executive produced the Netflix series, "Prank Encounters."