Gatwick to Paddington?

Quickest and simplest way is to take a Thameslink train to Farringdon and then the Elizabeth Line to Paddington. It will take a bit over an hour and you can use contactless to pay. Don't even think of a road transfer.

From the north terminal you need to get the free shuttle train to the south terminal. From there get a train to Victoria Station. You’ll need a ticket. Don’t get me Gatwick Express- more expensive and 3 minutes quicker

At Victoria take the Victoria line northbound on the tube (subway) and change to the Bakerloo line at Oxford Circus. This is an easy, level change through a short tunnel. The Bakerloo line goes to Paddington. You’ll need to touch in and out at the tube barriers using a contactless credit card or you can buy an Oyster card. (Former is easier and cheaper unless you have kids with you)

I echo the point about not even thinking of going by road

I'm in the Thameslink - Elizabeth Line camp. One less change, more modern equipment.

I take it that you are going to take a train west from Paddington station to somewhere like Bristol, Bath or south Wales? Should that be the case, consider taking a GWR train direct to Reading and then transfer to a westbound express. The GWR trains are usually every 30 minutes between Gatwick and Reading but the journey is slow going and the trains are old but it saves you the hassle of transferring via London and the fares avoiding London are cheaper. The journey time from Gatwick to Reading is pretty much the same via either route but I would rather look out at the countryside than go via London.

As said earlier, take the free shuttle between the north terminal and the south terminal to catch the trains.