A trumpet, not being a trombone (duh!) isn't capable of doing a natural gliss down a half an octave or so, so trumpet players 'half-valve', which means putting the valve halfway down so the airflow is very disturbed, and both the valve and the non-valve lengths vibrate (sort of!). This gives a very strangled sound which doesn't resonate at any particular note, making it easy to lip down and up quite a long way.

You MIGHT be able to get a bit closer by putting portamento on/off to a footswitch, sound to mono, hit the note and then hit portamento on, then play an octave (or fourth down, whatever) below, and let the portamento do the long bend down for you.

But, given that the tone as well changes radically, it's only a half solution. I don't think I have EVER heard this emulated well, unless you actually have a sample of the gliss. Kind of like those clarinet glisses that they have for the SA2 Clarinet.

I am right in thinking the SA trumpet doesn't have a lip gliss, aren't I?