I found another signed spur of the GRR in Atlas, IL. It is signed west along U.S. 54 toward "Pike Station River Access." This implies that the spur continues at least as far as 256th Street and maybe to Ralphs Landing Road or the Two Rivers Marina, but there are no signs in Pike Station or anywhere off IL 96 as far as I can tell.
I suspect there are more of these poorly but unambiguously signed spurs scattered all along the Illinois portion of the GRR in equally unlikely places, but I don't have time to look around for more tonight. I have only driven the stretch of the GRR from I-72 south to Mozier, so I can only safely say that there are not any additional spurs on that stretch beyond the three I have mentioned.
There's an incredibly short one to Riley Creek Rest Area, one along State Forest Road, Wolf Creek, and one heading into St Louis from the North along IL3
The only one signed with more than a sign at the diverge (that I've found) is the 'Free Ferry' one in Kampsville where there's supposed to be something the other side of the river, but there's not (in part because there's the Illinois River Road taking up sign space - see also other stuff being signed where there's supposed to be a GRR spur which isn't signed as it's not as important as the other scenic byway/trail/whatever).