The difference is that Zed has power in lots of different areas, while Gragas only has power in few related areas.
Zed is good at almost everything - assassination, pushing, taking objectives, and most of all, mobility and safety; Gragas is a tanky guy who is good at soaking up damage and disrupting, but doesn't do particularly much damage.
Zed has 6 skills and 2 passives.
He does high burst damage, high sustained damage, easy pushing and farming, strong tower-killing, a gap-closer initiate, and has extreme safety with two escapes.
Gragas has 4 skills and 1 passive.
He has slow damage, some sustain/tankiness, one escape which is also his initiate, and some strong CC.