Can i use gsn for any token in my CEX ( Centralised Exchange )?

Hi! by reading the documentation on opengsn; I was able to understand that to use the gsn, you have to modify the token code. What about tokens that already exist?
I want to use the gsn on my CEX to allow users to pay the gas fee during withdrawals with the token they want withdrawn. (ERC-20, BEP-20 …). How to implement this? since I will not be able to modify the source codes of all the tokens that will be listed on my exchange.
Best Regards

Tokens that already exist and do not support GSN cannot be used through GSN.

What other tool can we use in this case? to allow users to pay gas fee with tokens

If you're building a centralized exchange you don't need any special mechanism. Your CEX has custody of the user tokens, so you can just charge from there. You will need to pay for transactions in ETH, of course, but you can charge users for the fees that you pay.