The very best place to ask questions about Gmax would be here:
https://www.fsdeveloper.com/forum/
There's a Gmax-specific forum there that can help with any issues you come up against.
For your rotation problem, I'd say that you'll need to reset the pivot so that the pivot axis is properly aligned with the environment.
1. Make sure the part isn't a 'child' of another part ('
Unlink Selection' broken chain link' icon, upper left);
2. '
Utilities' [hammer icon, upper right of screen], then press '
Reset Xfom'. This makes sure the part is properly sized.
3. After that, '
Heirarchy' [icon is a square with three small squares underneath], then press '
Affect Pivot Only', "
Align to World', '
Transform' and '
Scale').
Then the pivot axis will be aligned with the environment in both 'View' and 'Local' settings. I use 'Local" to animate a part that is tilted, as then the pivot will follow the parts' new angle.
This may result in the part becoming bigger or smaller, but in that case just re-size it. You may have to redo the animation, as well.