check its not the linkage in the leg into the gearbox. there will be a small inspection cover, bottom bit of the leg, remove that and you will see the rod intot he gearbox. loosen that and check the gear lever at the top, if it moves freely now, the problem is in the box not the linkage
drain the gearbox oil and flush the box out. you should be able to grip the fear linkeage through the inspection port with pliers and try work it up and down to see if it will free
had a similar issue years ago on a johnson 40. gearbox oil was like paté
I ended up filling the box with diesel, spinning the prop and wiggling the rod, then flushing the box with hydraulic oil and finally refilling with gearbox oil. worked great for years after that