Depending on exactly WHICH FGC25 you have you will have a single large counterweight retainer bolt, or you will have 2 smaller retainer bolts.
The late models (FGC25K, and FGC25N) use a single large retainer bolt, I think it is 30mm diameter with a 46mm (1-13/16") hex head.
The earlier models (possibly FGC25 D up to the K models) had 2 20mm bolts and it seems like those bolts had 1-1/8" hex heads.
If you have the older model with 2 bolts, I hope one of the bolts has not broken its head off (leaving the remains of the broken bolt sticking out like a stud).
That scenario happened to me some years back that required us to use an arc welder to melt off the remaining portion of broken bolt before we could lift off the weight itself. It is possible that the bolts have fallen out already.
I also recall that those 2 bolt models had big issues with the ramped brackets that the counterweight rests on.
The problem with that was the counterweight bolts would loosen (and sometimes fall out) and then the counterweight would bonce up and down hitting the ramped surface of the frame brackets. The hammering would wear away a lot of material on the ramp AND on the counterweight. That would cause the counterweight to settle down further (in relation to the retainer bolt holes) and prevent your retainer bolts from entering their holes in the frame when you tried to remount the weight.
The solution for that was to weld up the worn away material from the ramped surfaces enough to replace what was missing from the ramps and from the counterweight.
If you have never lifted a counterweight off before, you need a heavy chain (5/16" links minimum, 3/8" preferred) that has a single grab hook on one end only, about 5 feet or longer.
After you get the propane tank bracket off, you will see 2 large holes in the top of the counterweight.
You drop the end of the chain down through one hole and using a coat hanger or wire, you fish the end over and up through the opposite hole in the counterweight. It may be necessary for you to remove the airflow grille in the rear of the counterweight (if it still has one) to manipulate the chain.
The counterweight weighs around 3,300 Lb. so you need another forklift, or an overhead hoist of sufficient capacity to do this job.
If we could post photos here I could show you exactly where the bolts are. You will need to have your eyes about 8" to 10" above the floor at the rear of the truck to see the bolt, or bolts inside their recess pocket of the CW.