Welcome to the TBN.
What does the manual say about engaging the PTO?
Do you have live power on the PTO, or do you need to engage the PTO with clutch disengaged, and is the clutch a double clutch? Important to know this.
I suspect you have the double clutch, that the PTO is live (running) while you can change gears when the clutch pedal is part way 'down', and you can stop the PTO when the clutch is all the way down.
Try, with the transmission in neutral, pushing the clutch all the way down and engaging the PTO.
Then, push the clutch half way down to see if the PTO continues to run the brush hog.
Outside chance if you have the double clutch, that the second clutch is rusted together.
Sounds like you are jamming the PTO into gear, but that shouldn't be a knashing of gears.