If flying with TG out of BKK, and flying after your status is upgraded, I suppose it shouldn't be a problem. If flying before your status is upgraded, it will be a problem as the staff at the check-in counter and lounges don't have access to your flight history enough to conclude that you already qualified for gold. If trying to get access to some other *A lounge, I don't know what their policy would be and think you'd be much safer with the letter in hand than to risk being turned away. I've heard of some lounges (SQ I believe) refusing entry unless you present the actual gold card (or letter) to them, but don't know if this is normal or just some one-off type problem.