They deploy with weight on wheels or wheel speed spin up
BUT if they are not armed (in my case) or you get a red Ground Spoiler message (in his case), then they do not deploy.
Gulfstreams are not like other aircraft, I have flown most Falcon types and I was amazed that you could land with the speed brakes deployed.
The wing is clean and you don't have any other leading edge devices to rely on to help you slow down. You need to get the weight on the wheels as fast as you can, hence the large penalty for spoilers being inop.
On a Falcon you don't even have auto ground spoilers, the plane stops perfectly fine without them. It's apples and oranges and I think that's what's hard for people to get a grip on.
Just an FYI, Dassault finally joined the 21st century with the 900EASy (and I believe 2000 EASY/7X) which now have "Auto Airbrakes" on touchdown.
And yes the plane does land perfectly fine with them manually deployed prior to touchdown. Stopping distance is only marginally affected if they fail to deploy after weight on wheels.