Greets! Just want to know if i may draw/paint on paper my assets, configure them with gimp and then use them in unity? Would that be possible?
Yes. Just export them from Gimp (or whatever, really) in an appropriate image format. If you are good at drawing by hand this is a great (if not fantasic) way to make your own sprites.
Just a tip: by default Unity will want to compress you images. If you want pixel perfection "there are ways". First thing you might want to do after moving an art asset into Unity is click on it, and check the Inspector. There should be options there where you can turn off compression if you want clear sprites.