nmcheck.gnome.org facilitating ads?

It's probably your ISP injecting the ad, yes. I do not see it, and I've checked from a couple of locations.

There would be no reason for the gnome project to inject ads there. First of all, it's not meant to ever be seen by a human, so no-one would ever see the ad, rendering it ineffective.

Second, the Gnome project doesn't appear to have advertising at all on their pages. They load no javascript external to gnome.org as far as I can tell. The hosting is provided, as you say, by Red Hat.

To stop this, you can either use a VPN connection, or petition your ISP to stop this stupid behavior.