1) You can develop coeliac disease at any age and 50 is actually quite a common age to be diagnosed.
2) If you go gluten free then you may not get a proper result. You need up to 4 slices of bread per day for six weeks.
3) If you go gluten free for a bit and then go back to gluten for the endoscopy then your symptoms may well be a lot worse on gluten than they are now (and also may be different). Many coeliacs have reported this.
4) Go back to the GP and ask if he can prescribe or suggest any medication that can ease the symptoms a bit. Also ask if he can flag your case as urgent.
5) Ring the gastroenterologist's secretary and see if you can get a cancellation or if you can pay to see him/her privately (may be about £150 for a consultation but you may be seen in a week or so rather than 3 months). (Even if you see a gastroenterologist privately he or she can usually put you on the NHS waiting list for an endscopy OR the private list - a private endoscopy is over £1000. You would go on the NHS or private waiting list nearly 3 months earlier if you had a private consultation).