Do everybody gain weight when taking gabapentin and cannot lose the weight?

Not everybody: It is a common side effect but certainly not universal. Also, the amount varies a lot. If you have a personal reaction like that, it might not be best for you but even then attention to safe exercise and careful diet choices can help for many. It's a balance of pros and cons. Finally, almost nothing lasts forever including weight. That said, it is very challenging for some. Choices can be rough!

Causation: In my experience, most folks who take gabapentin do experience some weight gain, and on occasion, significant gains. This is indeed one of the limiting aspects of this drug. It would help, if you want to lose weight, not to take gabapentin concurrently.