You’ll be happy to know the NDMC has set up open air gyms in and around the city available to use for free. You'll find some of those at Lodhi Garden (closest metro station is Jor Bagh) and Nehru Park (closest to Lok Kalyan Marg metro station on the yellow line).
Timings:
Lodhi Garden: 6am to 7:30pm
Nehru Park: 5am to 9pm