Hi. From your post, you are indicating 2 issues.
1: The reel remains in freespool mode, the button not coming back up when you want to reel in, from a cast, so the handle just goes round and round.
2: If engaged, the reel reels backwards at all times, no anti-reverse.
Solutions: 1: Replace the position holder.
2: Replace the anti-reverse ratchet.
The TGC does not have instant anti-reverse. It's mechanical.
There is no specific tutorial for this model that I'm aware of. But if you follow this one, you will get the idea.
Abu 6000C rebuild - SurfTalk Oh. The forum appears to no longer permit outside links.
So, you need to google "Abu 6000C Rebuild" which will take you to the tutorial.
time to shout: PRINT A COPY OF THE SCHEMATIC FOR YOUR REEL, AND STUDY IT FOR A LITTLE WHILE FIRST, BEFORE YOU START UNDOING STUFF !!!!
Good fishing and fishing. KD