Much depends upon the gliders available at your nearest club.
Most Glass/modern two seaters can accept up to between 220 – 240 lbs maximum cockpit weight per pilot although this normally includes a parachute. Older machines will usually have lower limits.
The best answer is to check on the nearest club to you – via club search on BGA website – and give them a call or visit to determine the limits applicable to their gliders.
Most Solo gliders have similar maximum cockpit weights hence I would suggest that you will require to lose some weight before taking up the sport.:)
Proving that your diabetes is adequately controlled such that you meet the medical requirements for a DVLC Licence then you should have no problem with the gliding medical requirement.