It sounds like you are starting FSX with the engines already running, so when you load the 777, it moves forward and through the terminal.
I always start it with a cold and dark setting, so when the 777 loads and if the throttles a fully forward I press "F1". It can't move from where it's parked.
If the gear lever is up, I get a warning for that, so pressing "G", fixes it. It's not like the gear will retract and the 777 will drop to the ground. Just a simple warning horn goes off is all.
For me, the 777 does not always have the throttles fully forward when it loads. It only happens sometimes. I guess that because I always start with a cold-n-dark setting, it doesn't cause any problems for me.
If you are not loading the 777 into FSX without the engines running (cold and dark), so that the problem is happening when you start the engines, then perhaps you need to revise your before engine start checklist. ;)