Regardless if you can afford to buy sand or not, remove the gravel completely. You don't have to have substrate but you do need to remove it. If you don't your axie will swallow it, it can block him/her and at worst lead to a prolapse and death; or spend the next year/s constantly swallowing and pooing out gravel which won't be good for its body at all.
Our adults have been pooing gravel for over a year since they've been on sand.
Damn this must have been one thing you didn't ask on your list of ?s

otherwise i would have told you to not use gravel unless it was permanently siliconed down so it couldn't be swallowed.
Did you manage to buy any test kits for your tankwater testing? As your tank won't be cycled and your axie has fungus, possible pending temperature problems, you have to be pretty good about cleaning uneaten food and doing daily 30% waterchanges since it isn't cycled; otherwise your axie could get sicker than just having a fungal infection and would need to be put in the fridge for recuperation.
I guess you must have just been excited and rushed out and got everything after your parents agreed to it.
That may be the case just make sure you try and look after her/him as well as possible, even if it means removing gravel, daily waterchanges and if need be fridging her/him. Maybe you shouldn't have got an axie at all if you family aren't happy at the idea of it being fridged. This works better and sometimes cheaper than forking out for fish medicines. My mother freaked out when she visited and opened the icecream container in the fridge thinking I had leftovers only to be introduced to 2 of our axies. Won't touch any plastic containers in our fridge now

but she has warmed to our axies.