Unfortunately, this depends on the merchant. They set the amount that needs to be pre-authorized in order to get gas, which can be up to $100. You can keep more than $100 in the qube just in case, or you can check out this list of gas stations that have a pre-auth of only $1. (This is not sponsored by Qube): https://firstquarterfinance.com/gas-stations-that-only-hold-1-dollar/
Another solution is to go into the gas station and ask them to put $___ on whatever pump you are on! This will bypass the pre-auth amount, and you can spend only as much as you have in your gas qube!