And that’s the root of the problem answering this question, I don’t know if what you’ve been doing has made you ready.
If you’re confident that your technique is correct and that you won’t degrade to an unsafe degree in whatever workout you’re about to attempt, you can give it a go.
Be prepared to be very critical of yourself and if you feel that you’re struggling at all, swap to another movement.
Something OPT says is that TTB are his preferred sub for GHD sit-ups. If you can’t do the workout using TTB, you’re probably not ready to use a GHD.
Aside from mechanics, whether you’ve prepared sufficiently to avoid rhabdo is very difficult to tell.
Hoping one of the Brand X trainers will spot this and explain how people would be introduced to them at the gym,