We don't have any evidence that the Guardians are doing anything about the Reach, but given how little information we have about the inner workings of the GLC in general and insight into what the Guardians are thinking, we wouldn't expect to have any evidence. I talked about this a few days ago, but I'll expand here. The Guardians could be doing something like building powerful weapons to fight the Reach or enhanced their existing Lanterns. They could be making fleets of Maltusian tech ships for civilians to crew. Oa is a planet, and the GLC is only visibly using a small part of it. They could be getting ready to turn Mogo into an unstoppable battle station. We don't know.
Or, they could recognize that the Reach's ability to wage war is expanding faster than theirs, and go another route. They could, as I mentioned before, be socially engineering the rest of the galaxy to eventually fight the Reach as a unified whole. They could build malicious AIs designed to sabotage the Reach in any number of ways without their knowing it until too late. They could be encouraging internal strife so that the Reach empire devolves into civil war.
The important thing is that given our current level of knowledge, we wouldn't expect to know if the Guardians are doing any of these options or even all of them. The Guardians are secretive, but we generally know they're intelligent an well intentioned. Extrapolating from a lack of information that we would expect to have regardless of what the Guardians are doing is... Bad speculation.
Or, maybe the Reach just aren't as high a priority as we think they are. The Guardians may consider Darkseid to be a much bigger issue, or something else entirely.
Finally, WHY are you TALKING like THIS?
