I make a new file with a transparent background, add the text to that, then duplicate it for however many frames I want the text to be on the gif. Like, if the gif is 100 frames and I only want text near the end, I might just duplicate it about 25 times? Then just select the frames in the gif that you want, select the whole text gif and copy it, then past it over. I hope that makes sense and answers your question!