Dual control of relay
For relay controls, you generally associate ORs with parallel wiring and ANDS with series wiring.  So can you simply wire the GPIO and the USB power supply in parallel to the relay coil?
Probably not.  They are separate voltage sources and mingling them will have unpredictable unintended results.  With relay controls, a simple answer is to add interposing relays:

The USB power supply to the coil of one interposing relay
The GPIO to the coil of another interposing relay
The contacts of the two interposing relays in parallel between a power source and the control relay coil

Now you can select the interposing relay for the USB power with a coil suitable for USB power, and the interposing relay for the GPIO with a coil suitable for logic-level power, likely a solid state relay, and it may require a power supply separate from the contacts and coil.
The NO contacts of the control relay will then close when either the USB power supply is on OR the GPIO is high.
You'll need a power supply suitable for the coil of the control relay, and the contacts of the interposing relays must be rated so they're compatible with the coil of the control relay and that power supply.
This is a bit convoluted, which is typical with relay controls.  It may be better to achieve the control by wiring the power supply to another GPIO and using the RPi's logic to do the OR, and switch another GPIO that operates the coil of the control relay.  But there is a trade-off, you have to have access to the RPi's code, the ability to modify and maintain that code, and have the IOs available to do what you need to do.