I have what the OP wanted, a grease gun that runs on Ryobi batteries.
I acquired a used Lincoln grease gun, from a tool repair shop that I was working part time for. It had some minor damage to the case, but only really needed new batteries.
I cut the battery receptacle off of an old Ryobi flashlight, (the kind of flashlight you get when you buy a kit with several tools). The light just happened to be the original Ryobi blue color, which was similar to the blue on the Lincoln grease gun.
I cut off the part of the original handle on the grease gun that held the battery, and sanded the Lincoln battery holder until it fit inside far enough to glue it.
After soldering the power wires, I glued the two parts together with some JB Weld.
This left me with an almost undetectable hack. It looked so good, I took it to the tool repair shop and said, "Hey look, these Ryobi batteries fit on this Lincoln grease gun"!! Had them going with that for a while.
Even though the tool originally had 14.4volt batteries, I have run it on my 18 volt batteries for many years. It works just fine.
I can highly recommend the Lincoln grease gun, even without the Ryobi batteries.