1GW at 500kV three-phase AC is 1154 amps; 1 billion divided by 500,000 divided by sqrt(3).
You could handle that with one set of 1272kcmil aluminum conductors, or two sets of 300kcmil conductors, based off of this wire submittal: https://www.prioritywire.com/specs/acsr.pdf
The voltage drop will be higher than HVDC, but AC transformers are probably an order of magnitude cheaper than HVDC switchgear. That’s the main issue with HVDC, interrupting HVDC current is very difficult since there’s no zero point like with an AC sine wave. High voltage AC breakers use SF-6 to extinguish the arc at the zero point, which happens 120 times per second at 60 hz (100 times per second for 50 hz)
The market seems to be settling on a 5 cable setup and 2GW power transmission (Tenet & other European TSOs have a 2GW Platform build out…).