Is GTA Online cross-platform?

No. No combinations of those five platforms use the same servers in a way that would allow players on different systems to play in the same lobby.

Part of the issues involved with doing so include the fact that current generation platforms (Xbox One, PS4, PC) allow 30 concurrent players at a time per lobby, while the previous generation allows only 16.

Another issue is that, reportedly, there are policies among the console manufacturers that are highly against cross-platform multiplayer. At least between each other.

So as it stands, each of the five versions of GTA Online are separate from the other four. The closest you can get to cross-platform is by buying the game on a current-gen system, and transferring online character progress from previous generation version. There is also the a slight caveat that the XBox One is adding backwards compatibility for 360 games. It should be noted that in this case you would be able to play the Xbox 360 version of the game on either console with other people playing the Xbox 360 version of the game on either console.

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.