I recently got a 2006 Gas Gas txt 300 pro. The manual says to check the gearbox oil by unscrewing the sightglass and using the dipstick. There's no way this can be correct. How do I go about checking this oil level?

If you can see the oil half way up the sightglass, with the bike level, you are at the correct oil level.

Best thing is to change it, that will give you a base line on the glass.