If you're flying out 10:30 AM next day, I think you may have trouble. The check ins are long due the new anti terror regs: if you have to be in Gatwick from out of London this would mean getting out of Victoria at some ridiculous time like 6 AM or even earlier. I am not sure what the Gatwick express is like these days (it used to be quite good) but I don't know what time the trains start.

If you are in central London to see a play, then I doubt there's going to be many trains coming out of town to Gatwick at the time the play finishes. If it's a 7 PM start, and the play runs to 10:30, you will maybe only just get enough time to get out of the theatre and get a drink before the bars start to close: you'll then have to get the tube across to Victoria to catch the express.

I think your best bet would be to look at the times of the Gatwick Express trains out of Victoria station before making a decision. Don't underestimate how awful public transport is in London at the moment - a large number of tube stations are closed due to safety issues caused by the Fire Brigade strike - you may find the tube station near the theatre is closed!