I hope bitgo wallet gives users private keys? If Yes just import your private keys into trust wallet or atomic wallet, only centralised wallets that gives user no private keys are the ones that makes their own gas fee and they can't be adjusted
It is an exchange so they ae not giving you the private keys, they collect funds and you can withdraw them as you wanted.
without significant investment,we cant hire someone to create own wallet.
Wallets also significant if you want to run an exchange successfully so building your own wallet should be your priority instead of luring more customers now.