The largest crypto exchange in India currently is Wazirx based on liquidity and their partnership with Binance. I guess Zebpay is next. In terms of OTP platforms, LocalBItcoins is the best and I feel it is better than the exchanges too as long as you are dealing with right dealers.

Crypto casinos are legal since they never banned them though gambling with FIAT is illegal in most parts of India. Also, op has clearly mentioned that crypto like BTC, LTC, ETH, BSV and EOS are supported in the main post.