Depends what you consider a gamble. Players tend to have career arcs and Mbappe would seem to be on an elite one...doesn't mean he's guaranteed to be elite but but it's unlikely he's not at least rather good. See something like Draxler (who was on a similar arc to Mbappe at his age you could say) as a not ideal scenario for Mbappe.
Given his age and talent, £100m for Mbappe is likely less of a gamble than £100m for Griezmann, and you could argue it's a similar gamble to Mustafi for £35 (paid off) or Xhaka for £35 (probably not a big overpay, but jury's out).
I agree with @Craigie , it's more a question of perception and perhaps a conservative mindset (due to being stung maybe by the experiences of Ox and Wilshere?) of not wanting to place too much weight and expectation on a very young player from Wenger.