I am trading a gym membership with a Vendor to pay down an invoice. How should i track the credit? I added as a payment but not sure what account in should go into.

This is bartering.  There a lot of mean to record it, however, the easiest is to receive a payment in the amount of the membership and apply against the invoice balance.  When making a deposit, drop down a line and add the expense account used to track gym memberships and entered the same amount as a negative so that the deposit total is zero.

This is not what happens, for Barter: "When making a deposit, drop down a line and add the expense account used to track gym memberships and entered the same amount as a negative so that the deposit total is zero."

For Barter, you need to understand that you are tracking what you Owe the vendor for, but instead of paying in Funds, you bought something on their behalf.

Like this:

You pay for 6 memberships, but 1 is not your business cost. You post that as Other Current Asset = not for your business, but this other party.

Now, you enter a bill for what you owe them, such as Janitorial services or whatever it is you need to pay them for. The point now, is, the Bill is not going to be paid by Money. Put a Negative on the next line and post the same Other Current Asset. This way, the bill is 0 due; you cleared the Other Current Asset balance, and you have an entry that captures for the vendor name the Gross expense that you "paid" them for, just not as money.

And you should stop this sort of trade. A Barter is always the same as money, and in reality, you would never buy a membership for just anyone. That isn't a business activity at all. A Barter is more typically done because, for instance, we are customers and vendors to each other, both ways, so Barter allows us to swap the values for legitimate business activities while avoiding unnecessary Banking.

And now you have Vendor Reportable 1099-Misc Box 7 activities. You don't get that from using a negative on a Bank Deposit.