Yes! Please consume gluten starting NOW before your endoscopy. A gluten challenge for an endoscopy to obtain intestinal biopsies is between 2 to 4 weeks. Better to be safe as do more than two weeks in my non-medical opinion.
Eating more than two slices of bread is not necessary for testing, but it might be the time (if you can tolerate it), to load up in case you might have to go gluten free for life!
It is good that you are trying to determine the cause of your GERD and other issues. Keep us posted and welcome to the forum.