Both GMC Mumbai and GSMC Mumbai are poles apart in terms of academics, extracurricular activities, and campus. Academically, GSMC is definitely better than GMC, as most toppers in Maharashtra prefer GSMC over GMC, so the crowd is better. However, GMC is the central medical college of the state so it has the best faculty.
GMC has an excellent extracurricular culture with many cultural and sports festivals organized yearly. The institute focuses on the holistic development of the students and inculcates many values in the students. Many GMC alumni are successful IAS officers, politicians, and social servants. GMC has a beautiful lush green campus that is much more spacious than GSMC.
Overall life at GMC is more fun than any other college in Mumbai. So choose GSMC if you are only concerned about academics but if you want to experience both, a fun-filled college life and good academics, choose GMC.