If you are on per diems, you are getting reimbursed straight up anyway so it shouldn't matter whether you use a gift card, points CC, cash back CC, cash, etc., you are just required to report where you are and for how many days and depending on policy deduct certain portions if a meal is provided for you on a given day.

If you are on actuals, your employer should have some sort of policy dictating whether you are required to use a corporate CC (and it could be nuanced like CC required for meals over $50), or if you simply need to submit receipts for your meals (or meals over $X).

I've had all variations of the above -- in my consulting world it was dictated by the client with our own firm's policies layered on top (with our firm's policies taking precedent unless the contract from the client dictated otherwise). In my current role we have a different set of policies but for food it's per diem so it's irrelevant what we pay with since it's a straight reimbursement (actually easier to do something other than the corporate card to avoid having to pay the corporate AMEX bill from your personal bank account).