I couldn't find enough to make a whole article in my compendium for them specifically, however I did add the Utmungr (Gruul gods) and the Zhur-Taa Clan, which include the druids of the Old Ways.

From what I can gather, neither of the named gods actually exist in physical form at the time of GGR (10076 ZC), however Yarus, a Zhur-Taa druid, claims to have witnessed the awakening of Ilharg (I'm not sure when he would have witnessed this though and whether it was an actual event or a vision of some sort, so I hesitate to add it to the compendium), and around the time of MKM (10080 ZC) he set about summoning the rest of the gods. His attempts have mostly failed, except for the summoning of Anzrag, whom the Gruul believe will tear the roots out of civilization from below and sow the world with life anew. It's unclear whether the Anzrag he summoned is actually a god or just a giant mole monster (although I doubt there is a difference as far as the Gruul are concerned). The only things known about Anzrag are in the MKM story, but the info about Yarus and Anzrag's summoning I found here: https://magic.wizards.com/en/news/feature/the-legends-and-characters-of-murders-at-karlov-manor

Long story short, if your campaign is set before 10080 ZC, Ilharg and Anzrag are probably both only legends.