With Jet2 operating Boeing 737/757 it is not the ideal aircraft to fly into GIB
The 737-300 should be fine as Bmibaby used it on the EMA-GIB route about 5 years ago. Albeit certification may be required for some aircraft. I remember when second hand Monarch aircraft arrived they were not able to operate to into GIB when they first came, it was only a few Months later that they were able to run the route.