15.5 hp. Only is using 4 gears out of 6. what is wrong?

Need more information that this? Hilly? Age and condition? Been repaired before? Just got your learner's permit? Traveling to Idaho by Lawnmower to see your old brother before either of you die? WHut?

I don't know much about that model. If not a hydralic transmission (some are) then I would be gettting some oil change ramps and driving it up on them as far as it will go to check out the idler pulleys and the condition of the drive belt(s). Chains stretch, not a lot, but they do. Belting material stretches and wears, a lot and they do. Your idler pulley bearings/springs may be shot or stretched. Since you're saying the loss has been gradual, I'd look to these causes first.
I'm pretty sure you've checked the transmission fluid and the connections as well.

Wish you the best on this - doesn't sound expensive to fix yet so you've got a good chance of a cheap fix.