Sure people do it. Disney has taken steps to limit this so that parking at the resorts is not overburdened (when it's busy, they don't want to run out of spots for hotel guests).
There is a gate and guard at each resort you mentioned. The guard will let you in if you have business at the hotel (dining reservation, meeting, just want to shop), but will give you a temporary parking pass good for a limited # of hours. FYI - I don't think anybody is checking these except if the lot gets full, and, you don't need to show it to get out so it is a system that sometimes gets abused.
That being said, the GF self-park lot is far away (I'd rather park in the main lot and take the tram). The CR north garden wing lot is the closest place for a guest to park and walk to MK. Also, I think anybody who want to can pay for the valet parking at the resorts. There is no time limit on the valet parking, but it does cost more.