The GM shows to me as having logged in 2 hours ago (from this post). The first step, I'd say, is to directly confront him in PM about it. Don't be rude, just ask what's up and see if he has a good reason. From there, you have a few choices to make.
If he gives you a snarky response, doesn't respond at all (and keeps logging in periodically), fails to address the problem (assuming you presented it correctly) or gets back on track and then promptly goes back to not doing anything, you can make a roleplay inspired by what he created and not have to worry too much. I don't suggest taking his thread as it's ultimately his unless he actually gives it to you, but there's no rule stating you can't make something similar, especially if by all appearances you're in the right.
If you send a message and he simply doesn't log in again, it's possible something nasty came up, in which case I'd say wait for a good while, or if you're impatient, make something based off what I stated above.
But ultimately, the first step is directly communicating with him about the problem if you haven't done so.
