In the past I booked gc's and a seat upgrade to main cabin.
All 3 charges look similar on my statement (saying American Airlines TX).
The gc's were reimbursed 4 days later and the seat upgrade 3 days later. The seat upgrade was more than my remaining $50, but they properly capped the reimbursement at the correct amount.
It appears Amex treats all charges of less than $100 at AA (or other airlines) similarly and all are good for the airline travel credit. They likely can't differentiate the difference between a $75 upgrade, a $75 flight or a $75 gc. So it appears they just reimburse airline charges of any amount up to $100.