You can link invoices to payments: when adding an income transaction, you can select the invoice that the transaction is a payment for.
I don't recommend creating an extra user for this; users should represent real people. You could create an Accounts Receivable fund instead and log the future income as going into the fund, then log a withdrawal from the fund when you actually receive it. But that requires extra steps and is a little misleading because a fund is meant to show saved, not expected, income.
I would simply log the income transaction on the date of the gig. This would let you document the shares before you forget them and show your bandmates that the money is coming. The income will show as Owed on the Finance > Totals page until you receive the money and distribute it and log that as a distribution transaction, at which point it will show as Received.
If you want to track whether you have received the income yet, you can use the Paid checkbox on the linked invoice. Or if you're not using invoices, you could leave a note in the Notes field on the transaction.
I'm on the fence about whether you should post-date the income transaction. You could, but I don't think there's much benefit to doing that. I think I would rather show the definite date that the money was earned rather than the anticipated date that it will be received.