Welcome to TBN. Great part of the country you live in - my aunt and uncle lived right outside of Phillipi on the road that runs up between the highway patrol post and P-B HS. Many fond memories of staying up there with them and going to the Barbour County Fair when I was a kid. Even saw Larry Groce there singing his one big hit back in '76 or '77. :thumbsup:
Does your Model-L have a two-speed axle? If so, there should be a lever (not one of the ones for hi/lo, forward/reverse) that runs to the right side of the axle (as you're standing behind the machine). That point behind the wheels gets a lot of gunk/dirt/grease/etc. and can mess up the engagement of the axle.
When you say you're engaging the PTO, are you also engaging it with the lever on the left handlebar? That lever also needs to be engaged for it to be in gear. So, leave the PTO unengaged, press the right lever forward (to go forward) and then engage the left lever forward for "hi" or backwards for "lo".
Is this Gravely new to you?
Hope this helps.