I can use the console to add this perk manually. But how to make it so that code checks and uses light armor gauntlet value?

This doesn't sound difficult but I know nothing of modding.

I don't know Fists of Steel but I believe you want to change them to count as Light Armor?

This can be done with TESsnip. Search for a Tutorial how to get it running.

If I remember correctly it was the BODT var you have to change.

Fists of Steel is a perk, not a piece of armor.

I believe it's implemented with a script, that checks for your currently equipped gauntlets and adds a matching Fortify Unarmed effect if they're heavy armor. This is not a simple data record that can be changed with TESsnip, it's compiled code... you'll need to wait for the CK to change this.