It depends if you are just trying to gain pounds, or if you're looking to bulk up, and build muscle.

There are many things to take into account when try into gain weight through exercise. Are you doing the right type of exercise? 

Are you fuelling your body to perform the workouts needed to build your muscle. 

Is your diet giving you enough calories to sustain weight rather than losing it when you are exercising?

Abs is nothing other than your belly portion. In fitness world, abs is divided in to 2 parts lower abs and upper abs. Honestly speaking abs is the most difficult body part to bring to shape.