Cheap way to isolate and charge gel cell- relay or diode ?

Perhaps I do NOT understand the question.

Gel batteries are OK at a HIGHER voltage than wet cells. So, unless you are really over-charging the other bank (wet-cell or AGM), I don't see the issue.

Yes, this is from an old Xantrex inverter/charger manual, so recommendations may have changed.

SO, best advice is to post ideal bulk/acceptance and float voltage for both our battery banks from the battery manufacturer. Also, the voltage output of your "not smart" converter.

That way we are all dealing with FACTS.