Is GTA Online cross-platform?

The only exception to a universal No answer would be in regards to Xbox One's backward compatibility feature.

This feature (available since November 2015) allows you to play Xbox 360 games on the Xbox One. This includes multiplayer features so that you can play against other players using an Xbox 360. More details can be found from the official site here:

Xbox One Backward Compatibility

To avoid confusion, when you play an Xbox 360 game on the Xbox One console you can play against anyone that is either using an Xbox 360 console or the Xbox One backward compatibility feature.

Here is a relevant question from the feature's FAQ:

Can I do multiplayer gaming with friends who are playing the game on Xbox 360 or Xbox One?

Yes. For games that offer multiplayer gaming, you'll be able to play with others who own the title on Xbox 360.

GTA V was not on the initial 100+ supported games list. The list of supported games can be found on the official site here:

Supported Games

Note that more games are due to be added over time through future updates. The warning at the top of the current list is as follows:

Initial titles with more to come. Games are subject to change.