Gyno is short for gynecomastia, which is an enlargement/growth of breast tissue. Men have breast tissue, but very very little. You will know if you have gyno if your chest looks like you have female breasts, even if they are misshapened.
Fat is part of the gyno, but if you are running a cycle and you are getting fat deposits specifically in the pectoral area (also look out for tenderness/itchiness/swelling of nipples)then you are getting gyno.