Getting the list of traded coins or pairs for a specific exchange?

Yes - use the /exchange/{id} endpoint to retrieve top 100 trading pairs available on an exchange. Volume is returned in BTC and you will have to convert the data on your own.

Example: https://api.coingecko.com/api/v3/exchanges/gdax

Can I get a list of tickers on an exchange for a specific coin?

Yes - use the /exchanges/{id}/tickers endpoint, and specify both the exhange_id and coin_id.

Example: https://api.coingecko.com/api/v3/exchanges/gdax/tickers?coin_ids=bitcoin

Can I retrieve orderbook data from crypto exchanges?

Yes - use the /exchanges/{id}/tickers endpoint and enable the depth parameter. Specify the coin_id for tickers specific to a particular coin.

Reference: https://www.coingecko.com/api/documentations/v3