Ok, look at the parts breakdown for your deck. 72 or 60? The gearbox has an input shaft, which has the drive shaft from the transmission. Then it's got an output shaft. It runs from the gearbox down through the deck. That's the one we're going to focus on.
The output shaft doubles as the center blade's spindle shaft. There is only ONE bearing supporting the bottom side. Thus, it's not uncommon to have a failed lower center spindle bearing, then the loose bearing lets the shaft walk around, which takes out the lower gearbox seal. Very common. The fix? Remove gearbox from deck. Replace the lower bearing and seal, also the upper seal in the spindle housing/deck. Remove the upper cap from the gearbox and inspect the gears. IF they're damaged, rebuild it. If not, replace the lower seal, refill gear box and then reinstall it.