If you bought $1000 worth of GME with $1,000 then you are not on margin.
If you bought $1,000 worth of GME with $250 then you are on the max 25% margin, and you have 3 days to transfer $750 else position will be closed at prevailing price.
The concerted effort by all brokers will probably lead to a mass sell-off which any traders/ investors would have visualized by now.
Note: I am not involved in GME