Flight connection at Gatwick

The UK and Ireland are part of a Common Travel Area so you need to go through immigration when you arrive at Gatwick from a country outside of the UK/Ireland. You also will go through security (this would be true even if you did not have separate tickets).

Personally, I’d want more than 2h15m to accommodate a potential flight delay on the inbound flight. I’d want to reduce the risk of having to buy a new ticket.

Here's the fun part.....

you will go through UK immigration and security at Gatwick.

If you are traveling with a US or Canadian passport, you will also go through immigration in Dublin.

The CTA in Ireland only applies to British and Irish subjects.

Make sure you know and follow Ryanair rules to the letter or it will cost you plenty.

we are arriving at Gatwick

From where?

joe32F We are flying from LAX to gatwick

International flights connecting to the UK and Ireland

If you​ a​re arriving at London Gatwick from an international destination and are connecting onwards on a separate ticket, you will need to go through immigration. If your hold bags are "through checked" or you’re travelling with hand luggage only, you can then go through security to your connecting flight.
If your hold luggage is not "through checked", you must pass through Passport Control and then collect your luggage from Baggage Reclaim. Once you have collected your hold luggage, you can then check in to your next flight.
https://www.gatwickairport.com/flights/flight-connections.html

Thanks everyone. As I am "risk-averse", we've decided it's just too tight a connection and will take the later flight. It leaves us a 5+ hour layover, which is brutal, but maybe we can find some decent food near the airport and get out of the terminal for a few hours.

For the layover, you could look into purchasing access to one of the lounges.

More comfortable than the general waiting area, free food and drink, good wifi, etc.

Gatwick Lounges

Horley is the closest town to Gatwick. So close that you can see the end of the station platforms from Gatwick Railway Station platforms. When I lived there I was never sure if it was fastest to walk home or wait for the train.

While mainly residential there is a Waitrose Grocery Store there (Waitrose are a 'posh' grocer) and several nice restaurants including a Turkish and an Italian Restaurant.