I'm 26 yrs old. Is this gynecomastia or fat? (photos)

From your photos you do not look obese and appear to be close to your ideal body weight. Your breasts are very prominent and I doubt that diet will really cause any significant reduction. As such, this does appear to be gynecomastia. The good news is that it is treatable with minimally invasive techniques. Examination by a reputable plastic surgeon would be a good place to start.

Thanks for the question, if when you squeeze the tissue it feels soft its probably fat, if it feels harder than its probably some extra breast tissue which happens from time to time in men, you should see a local surgeon to make sure there are no issues causing this and to discuss your options!