Shaykh, will I be rewarded if I go to the gym with the intention of keeping my body fit and becoming a strong believer? If so, would that make it recommended (mustahabb) or would it still remain merely permissible (mubah)? Also, is going to the gym considered a waste of time for a practicing Muslim?

Going to the gym is not a Sunnah. Strengthening your Iman, increasing your knowledge, and increasing in acts of worship (voluntary acts of worship) is what is favored. As for being strong, it is something that is good as long as it does not involve something that is forbidden or take you away from the Deen of Allah.