We talked about feeder workouts in this thread.
“GPP” is a term that’s super-misused and, basically, entirely inappropriate and irrelevant to anyone who’s not an athlete or competitor, so I’m not sure what you meant by trying to do “a GPP style rest day.”
Active recovery, doing some kind of less intense training, is great on non-lifting days. What you did with 1 not-killer set per bodypart is fine, but it would probably make more sense and be more practical to not have to go to the gym for that brief a session (unless you train at home, in which case nevermind). Doing some super-basic bodyweight stuff could get essentially the same job done.
A ton of articles talk about other active recovery methods.