The National Rail website is showing "It is not yet known how Gatwick Express
services will be affected by engineering work from Saturday 7 to 13 June 2014. Please check back nearer the time of travel". (That was based upon searching for services on 12th June, as you've not provided an exact date). However Gatwick Express (which operates from Victoria) isn't the only service to the airport. The First Capital Connect services (which call at London Bridge) appear to be running normally. So the alternative route (as offered by the National Rail website) of taking the Northern Line to London Bridge seems perfectly feasible.
However, as you seem to have realised, it's actually easier to catch the First Capital Connect services at St Pancras (simply by walking over the road from Kings Cross). The National Rail website again shows services running normally from there.
So I'm confused by your post, as it seems to be the 'wrong way round'. It's services from Victoria which might be affected by engineering works (so that's the station you might need to
avoid, rather than to get to!) whereas services from St Pancras appear to be running normally.
If you enter Peterborough to Gatwick on the National Rail website it offers travel on both Gatwick Express and First Capital Connect services, with discounted 'Advance' tickets (typically at £22.50) offered on both routes. Theoretically travel on First Capital Connect should be by joining their service at London Bridge but I can see no reason why you couldn't board at St Pancras instead. (Indeed, I've done exactly that myself with no problems!):
http://www.nationalrail.co.uk/
If you decided to take the National Express coach to Victoria Coach Station it would seem silly to make the trek to Victoria Rail Station (which can be quite arduous with heavy luggage) when there's an hourly coach service to Gatwick from Victoria Coach station anyway, with through fares available from Peterborough.