You dont need to worry, because migrate export is never done on gateway, only management, so it would stop mgmt access for probably few mins. If you wish to be on safe side, you can simply run something like cpinfo -o /var/log/cpinfo.out
Just to clarify, as some people get confused about this...if you do say cpstop on gateway, it will unload the policy. If you do SIC reset , it will load initial policy, which would literally block everything, except ssh and web UI, but only on port 443, thats it.
Hope that makes sense.
Andy