If a service member decides not to use their GTCC, but instead pays out of pocket, do they still get reimbursed with TLE?

I've been out of the loop for a while but recall it's not wise to pay your own on some stuff. The contracts GSA has with air carriers, rental car agencies, etc. have bennies like refundable plane seats, no charge changes, auto insurance (Government is self insured). So when your travel folk book you a flight, it goes on your GTCC. I found it easier to leave flights, rental car, and lodging on the GTCC as you had to have those receipts anyways. Everything else I paid out of pocket. That way when liquidating the orders, it was easy to figure out how much just gets pulled to pay the GTCC. I'd get bills that'd say zero payment required.
One thing about the GTCC though. Misuse invites a heavy handed reaction and many times overreaction. The Powers didn't go for the honest mistake thing very much.
The other thing is you may not have sufficient credit limits to do some things on your own. Even as an O-6 I chose to have my personal card set at $5-10K. Post 911 one "trip" had me doing $18K on air travel alone by the time I got back. That and I had to have a pile of cash for "things and places" that don't take plastic.
BTW, remember that reimbursement is up to the Government rate, unless actual expenses are authorized on the orders. Make sure if there isn't a GSA rate, etc. your orders are endorsed to cover you.

True

YES but need receipt