Does a tool like that exists for GMOD?

I know this may be in the wrong section but listen…

is theres some addon or tool which can help doing animation? Or maybe just onion skin? (I meant, you can see the last frame you took.)

No, But maybe you can request here: http://www.facepunch.com/forumdisplay.php?f=66

http://www.facepunch.com/showthread.php?t=1230140