If it was THAT easy to destroy a jump-gate, Gallia would've done it already to Magellan->Manchester gate in order to severe the ability to supply Bretonia. And they can probably muster far more fire-power in order to do that.
Leeds is currently under GRN control, and while Edinburgh gate might seem undefended from an OORP perspective, I am more than certain it's well guarded. Anyhow - even IF that gate were to be destroyed, I doubt that would cut their supply lines. Worst case it brings the GRN assault into Bretonia to a small halt while they redirect forces to Tau-31 - something that will probably just end up making the whole place more secured and also put a HUGE amount of pressure on the imperial forces.
T