Two tools i always reccomend for Gmod are 'Ragdoll Mover' and 'Stop Motion Helper', at bare minimum
Ragdoll mover makes everything alot more precise and tweakable, including beyond what the models bones are usually allowed to do XD Stop Motion Helper is a tool that makes it much more like a regular animation program, since it allows you to save frames that the game can then transition and tween between. Unless its these tools making things buggy, i would say go with these two
As for Proper Video Software, I started out with Windows Movie Maker and Lightworks. Lightworks allows you to do stuff with Layers and a smidge more technical aspects, havent seen it in years though so im not sure whats changed
Happy Animating!