They both mean "hold firmly", but grasp can also be used for abstract objects like 'ideas' or 'thoughts'.
ex. O I've grasped the idea of grammar.
X I've gripped the idea of grammar. (except for poetic purposes)
Also, I believe the slight difference between grasp and grip is that grasp has a more recent, light, uncertain or unconcerned feeling, but the action is finished. Grip has a very strong affirmation in the action, and it's continuous. Therefore:
Grasp - a firm hold that just happened, and unsure whether how long the hold will last, or will need to last, but it's done.
Grip - a firm hold that's doing all it can to never let go.
Hope this helps! :)