Retail Girl is stating the policy correctly. The OP did nothing wrong because the original purchase was paid for with a REDCard. No, neither the 10% TM discount or 5% REDCard discount will apply to the purchase price of a Target Gift Card (i.e. if you buy a $25 Gift Card you would still pay the full $25), but that gift card can still be redeemed in conjunction with a TM discount.
Where people get in trouble is when they purchase a Target Gift Card with a 3rd-party debit or credit card and then use that same gift card on a transaction along with their TM Discount. It's one thing if you actually received the GC as a gift; that's allowed and it's why the system lets the transaction go through. But it specifically states in the TM handbook that "You cannot use non-Target debit/cash/credit cards such as Visa, MasterCard, Discover, or American Express with your discount. It is not acceptable to purchase Target GiftCards with a non-Target debit/cash/credit card for the purpose of working around these restrictions."
That said, I know there are stores out there that ignore this rule. This doesn't mean you still can't get in trouble for it. Even if your AP or STL allows it, if a TM from your store tries this practice at a store that doesn't, that store's leadership can easily inform the DTL or APBP somebody will eventually take the heat. Just not worth it.