Aircraft is limited to .78M cruise with the gear door(s) removed.
Not any more; that was never intended to be a "limitation", and as of Jan 16th this year (for TC MMEL) it's been changed to more "advisory" type wording in a more recent revision, and I expect it to be removed in the near future. The FAA change may still be making its way through the system, though.
With the caveat, of course, that whatever paperwork you have right now obviously is the thing to obey.