Gatwick Express or Southern Rail

Yes.

I prefer Gatwick Express.

If the Southern Railway is on strike (industrial action), will the Gatwick Express still be in full service?

it varies depending on what the issue is, but usually one does not affect the running of the other except that displaced passengers may overflow onto some of the services, especially at peak times.

Thank you.

I am arriving at North Terminal.
All Rail/ trains leave from the South Terminal, I think. There are Bus/Coaches leaving from North Terminal.

Is it easy to use Southern Rail to Victoria Station instead of Gatwick express? Southern is much less money and you can use the oyster card . Which would you use?
In my two recent flights into Gatwick traveling to and from Victoria Station, I've used Southern Rail on 3 of the 4 legs and used my contactless card with ease on the last trip on Southern. Southern was convenient and less expensive, but I would have been willing to do the GE if necessary.