I'm starting with the assumption that the gasket for your windshield is the same as for the Seafari cabin windows, American Marine TP-139. If not, you can disregard the rest of this post.
Getting the gasket in is tricky. The part that is .444" wedges into the frame and the .672" part lays against the glass. The part of the .444" leg that slides into the frame under the glass (i.e, the bottom part in the picture) has to go in first. You have to really squeeze the base of the .672" leg to get the other end of the .444" leg to lock in.
I hope this is less confusing to read than it was to write.
Dave
Oh, yeah, and you may want to put a thin bead of silicone caulk around the frame before you lay the glass in. The gasket holds the glass in place, but it doesn't seal the window.