If this was indeed a stroke(which is common in older gerbils) make sure Thunder can reach the water bottle the water bottle. If he cannot feed himself, you can syringe feed some baby foods or Oxbowbow critical care which will help with recovery. Some cooked oatmeal is a good food if he is having problems with chewing. If he has problem with the water bottle, then an eye dropper or syringe works well to be sure he has a good fluid intake.
Giving him a quiet area in the tank by covering with a towel and keeping in a low foot traffic area will help with recovery. Stress will slow recovery time time, so keeping him quiet and comfortable is very important.
Strokes in gerbils will usually resolve on their own over time. Most Vets will not give any medication for a stroke, but give you the same info that I gave above. If Thunder starts to get worse or having more strokes, then at that time a Vet visit will be needed.
Oxbow critical care is a supplemental food given to ailing small animals: http://www.oxbowanimalhealth.com/vets/products/critical_care
The company also makes a staple food for hamsters and gerbils that is excellent to help with the weight and feeding. Many foods have a lot of seed and fat foods which does not allow an over weight animal to loose the weight.
Once he recovers from the Stroke I would recommend toys for activity to keep him active. that will help with the weight too.
The two best foods for Gerbils is Mazuri or Oxbow which offers a good balnced diet high in fiber, but low in fats.