Oxenfree has been updated to 3.0, then 3.0.1, then 3.1.0, the last in July.

Why is GOG falling so far behind on this?

Because unfortunately there are some publishers/developers which treat GOG customers as second class citizens.

The best thing you can do is to try contacting GOG support or, even better, the publisher/developer Night School Studios and ask them to update the GOG version.

Apparently a new version is out that teases Oxenfree 2

https://www.rockpapershotgun.com/oxenfree-2s-villains-are-sending-spooky-signals-to-the-original-game

I asked them on Facebook

Hi - heard about the new update teasing the sequel. Only problem is I own Oxenfree on GoG and there does not seem to have been an update for ages. Come on, give us GoG folk some love!

and they replied;

Night School Studio
We hear you Andrew! We hope to bring the patch to other platforms in the future. Thank you for asking!


So.... pretty non-committal, don't hold your breath!

Coincidentally I contacted them on Facebook as well and I received a similar reply:

It doesn't take 6 months to update a game, small team or not. Anyway, I finished 3 playthroughs without issue, so if it's just advertising for the sequel, I can do without it.