Applied for my baby's GE a few days after getting passport. got approved, but delayed the journey to the GE office until 5 months later..
We had flow from LAX multiple times but had thought we would need to wait for an appointment etc, and were not willing to come early and schedule for the day of a flight.
We were in costco right near LAX so we did a walk in - they did not ask if we had an appointment.
We said it was for our baby - they gave us a number and 5 minutes later we were in getting the photo. They said they no longer require fingerprints for baby's (under 5?).
All in all, if your in LAX, it's well worth trying a walk in. Miami, when i checked had available appointments almost all day every day for months - so flying through there i definitely recommend popping in for a walk in.
I think maybe we should make a Wiki for GE walk in friendly places?