In case it helps, this type of gasket is often called self-sealing two part weatherstrip.
Basically you have the "H" part (the size of each half of the H can be different, say if you have 1/4" fiberglass and 3/16" window material).
Once the window is installed with the H-strip (a piece of string helps, pull it out as you go), then you put in the second piece, which is a little triangle and is called the "lock strip." It basically wedges the H tight. There is a lock strip tool you can use.
School buses often (used to?) use this type of window gasket.
One manufacturer whose product I've used is CR Laurence.
Here are some of the "H" parts:
And a one piece variant with the lock strip attached: