This tip might help someone with a GT and a big helmet.
I have a Schuberth M1. It's open face and size XL. I've tried to fit it under the seat in the prescribed ways, upside down, sideways, but it's always a touch too big to close the seat. So I turned to the endless magic of the bungee cord.
I hung a three-foot cord on the frame tubes on either side of the trunk. I pull the cord forward, slide the helmet in facing forward, push it forward into the bungee as far as the helmet will go, center the helmet in the trunk, and somehow that math works perfectly. The seat closes easily every time. I drape a small microfiber towel over the bungee as a soft landing spot for the face of the helmet.
TIP FOR HOLDING THE SEAT OPEN. This applies to some older GT's where the lift arm has grown tired and the seat won't stay up. It can also happen with a backrest attached; the extra weight makes the seat want to close. I tape together three short paint stirring sticks (in photo just below the helmet). When the seat is open, prop the stick under it near the nose of the seat. There is a little lip on the underside of the seat that will catch it. Voila. No way your seat will slam shut on you unexpectedly.