That is one that is 'in the cracks' but here is my gut reaction:
If your contract states that you will be paid/reimbursed with a $40 gift card you really need to consider it a $40 payment and deduct your $25 reimbursement to find your 'profit' for the shop.
But if you contract for a $25 reimbursement only shop and the report indicates a wretched customer experience (such as we had at a bison restaurant) and you are paid the $25 and then sent a gift card in addition by the restaurant as a 'oops, sorry, try us again', the extra gift card is not a payment.
It is known that many of the time share places offer gifts to non-owners who listen to the spiel. Sometimes shoppers are required to refuse the gifts because they are being paid to do the shop and sometimes the shop instructions are silent on this so the shopper may accept the gifts. The gifts may be treated as gifts since they are not payment that has been contracted for. If the shop indicates that you will be paid $X for successful completion and will receive $50 in gift certificates for attending the meeting, then the gift certificates become part of your 'pay'.
Overall what I am saying is that if the gift card is what you contracted for (or part of what you contracted for), then it is income against which you can offset expenses.