2019. Which isnt disimilar to runway 3 at LHR
Yes it is. LHR could, theoretically, be given government approval for a 3rd runway tomorrow. LGW has a longstanding agreement which forbids expansion for another 11 years. The two are not remotely similar.
given that Gatwick Airport is surrounded by green fields in all directions
LGW has a large town to the south, residential areas to the north, and a motorway to the east. I agree that putting another runway in there, post-2019, would be easier than LHR but no doubt it would still encounter a huge amount of anti-expansion protests.