They owed you cash. You didn't get cash.
You are being partially paid for the inconvenience of being paid by a Visa gift card instead of the cash you are entitled to.
My friends often say "that item, plus $6, will get you a small coffee at Starbucks" to insult the valuelessness of the item. Actually, this thing, plus $6, will get you a small coffee + a $94 gift card + $6 cash ...
... if you normally pay cash at Starbucks. If you normally charge it, then you have to search for other places you normally pay cash, and use the value there in its place. Or, spend it instead of your normal card, then next month enjoy a lower payoff on the card if you pay in full, or a slightly lower monthly payment on the card for 30 years if you don't.
The inconvenience of having to pursue your money in that fashion is a reasonable reason for the increased value. Don't even begin to feel bad about it.
You could and should mention it to them. However, if you filed for actual reimbursement, you would need to assemble receipts and put it on a TPS report (make sure to use the new cover sheet!!), send it to the hiring manager for approval, then it goes over to accouning, they post it to the correct account in their Quicken, then they issue a paper check and the appropriate manager signs it, and a staffer posts it off to you. All this rigmarole will cost the company more than $100.
That is why they do $100 gift cards. It is business efficient.