This is the boilerplate language that's likely in your T&C for AMEX signup bonuses. Emphasis added:
The following charges do NOT count towards the spending requirement: fees or interest charges; purchases of travelers checks; purchases or reloading of prepaid cards; purchases of gift cards; person-to-person payments; or purchases of other cash equivalents.
So they can by rights say "no signup for you, we told you that in the T&C for the signup bonus" if your GC purchases are what qualifies you for a bonus. So it's up to you, I suppose.