No need for another jet.
The E195 operates 4 rotations daily to Gatwick and back. It is supplemented by up to 3 ATR72 a day on the route. This gives the required number of seats per day on the route. The Gatwick route makes a profit for Aurigny unlike most of its others to Stansted, East Midlands, Bristol and Manchester from GCI, which are all marginal with pax numbers only justifying an ATR 72. There is no requirement for an additional jet in the fleet as its load factor would be well under 50% most of the time.
PS Not true that Gatwick is only regular route: Man is X2 daily, others are at X1 daily except at very off peak times.