I used to work for GNC and if its a good store you should be able to just ask them they will point you in the right direction.
I would recommend a mass gainer protein, they have higher amounts of protein, complex carbs, and calories. You can mix whatever flavor you choose with 2% or whole milk and maybe fruit, ice cream, or peanut butter. Try different things to find what you like best.
As mentioned by others continue exercise and a balanced diet.
Current '04 CRF450R-Protapers, Stealth best-shot
'86 TRX250R-300cc fully modified and restored
Previous '07 CRF250R '05 YZ125 '05 CRF150F '04 CRF100F '01 CR250 '97 RM250 '93 KX125 '87 KX80