Be careful what you wish for with regards to anything even tangentially related to destroying gates. The Devs decided that Rheinland plans to coordinate the destruction of gates between the Rheinland/Lib buffer systems and Kusari/Lib buffer systems as just a possible backup plan would become Rheinland just blowing its gates, pissing off Ageira, and Kusari not doing anything - because apparently Liberty can only be protected from its enemies and not cut off from its unfriendly trading partners. - TLDNR - planning things like this is like making wishes with a monkey's paw.
Assuming that Liberty and Bretonia do use big fleets (counter to what you suggested) because if a small fleet could easily destroy gates, then the Gallics would have simply gone around destroying gates in Magellan to cut off Liberty supply lines into Bretonia long ago.
Even if the Gallics can't react quickly enough to protect the Edinburgh gate, they'd certainly be ready to cut off the Bretonian/Liberty fleet retreat - and/or launch a devastating counter offensive into a vulnerable New London.
