Gatwick Airport

• Customs is the check on goods. Checking the contents of your luggage to see if it contains any forbidden items or items for which customs duties (taxes) must be paid.
• Immigration (aka passport control) is the check on people. Checking whether you are allowed into the country and if so for how long. This is where your passport is stamped if necessary.

Assuming you are doing this as one through ticket.

The UK is not in the Schengen Area. At Gatwick, you will stay "Airside" in transit, and not legally enter the UK. So no Passport control, no customs. You will not see your bags as they will be labelled through to Milan. The airline will want to see your passport as an ID check.

At Milan, you will go through immigration for entering the Schengen Area, and your passport will be stamped if necessary. You will then pick up your bags, and carry them through customs. Customs is the one you need to worry about the least, as most people are not stopped, they just stop and check a small random sample.

if I fly into the south Terminal on Norwegian Air and fly out of the north Terminal on Easy Jet, am I still "airside"?

I have taken the shuttle between the north and south terminals and it is outside the secure areas of the terminals. I believe you will need to go through immigration, customs, and security at Gatwick, as well as immigration and customs in Milan. I did fly into Gatwick last December on the same Norwegian Flight from Seattle, which arrives at 7:00am. At that early time the terminal was not busy and there was a 10 minute wait to get through immigration. It may be longer depending on where you are seated on the plane (I was seated in the front of plane and was one of the first passengers to leave the aircraft).

I don't know this factually, it is worth checking, but I don't believe that either Norwegian or EasyJet interline baggage.

In that case you need to retrieve the bags and do the whole procedure at Gatwick, then be sure that your bags and you are checked in before the EasyJet deadline after moving terminals.

Then at Milan you need to go through Passport Control but because (at the moment) the UK is within the EU customs union you will walk through the EU customs door after you have retrieved your bags.

If you had been on a heritage airline combo, the scenario shown by Chris F would have applied.

if I fly into the south Terminal on Norwegian Air and fly out of the north Terminal on Easy Jet, am I still "airside"?

In this case it IS NOT one ticket, it is two. Easyjet DOES NOT do connections. This will be two entirely separate flights. Your bags will only be labelled as far as Gatwick.

At Gatwick you will go through UK immigration, pick up your bags and carry them through customs. You then need to get the shuttle train and go to the Easyjet check-in desk. Make sure you print out your Easyjet boarding card before you leave home.
Hand in your bags at the Easyjet bag drop-off and proceed to security.

At Milan you will go through Schengen Area immigration (another) stamp in your passport, pick up your bags but in this case you do not go through customs as both the UK and Italy are in the EU (until March 2019).

Have you checked the Easyjet minimum times? You must be at the gate when they say, if you are late you will be denied boarding. There is a large risk with this connection that if the first flight is late or you are delayed in immigration (1 hour wait is not unknown) you miss the Easyjet flight and are stuck in Gatwick.

Maybe too late for your journey but you can check your luggage for the Milan flight in the Baggage Reclaim Hall. So no need to take it through customs etc and then recheck it at the airline bag drop. If you have not already done so (can be done 30 days in advance with Easyjet) they will also print out your boarding pass.

https://www.gatwickairport.com/at-the-airport/flying-in/gatwickconnects/

List of airlines using the service, click on "Gatwick Connects"
https://www.gatwickairport.com/faqs/flight-connections/

Similar to what Linda has mentioned, we used the Gatwick connects service last year on our trip. We flew into Gatwick on Iceland Air and then took EasyJet from LGW to Athens. Easy Jet has the counter at the baggage reclaim hall, when you approach them with your Easy jet Boarding Pass they will tag your luggage and then hand them back to you. You will have to clear customs, and then a Easy Jet rep will be waiting for you to take your luggage.

Saves you to having to carry the luggage on the shuttle train between terminals and then drop of the check luggage. Worked pretty smoothly for us.

Also to clarify, we did not buy tickets through Gatwick Connection. We bought our own standalone tickets. As long as one of your tickets is on Easy Jet you can use the service for free.

Hopefully you chose the 145PM easyjet flight and not the 9AM