New Heathrow terminal

The new Terminal 5 that opens late March at Heathrow just brings together existing flights operated from Terminals 1 and 4 at Heathrow into one terminal.
BA flights from Gatwick still operate as before. A handful of flights are being swapped between the airports and some European destinations like Madrid and Rome are operated from both airports, the cheapest fares being from Gatwick.

The answer to this is going to be vague. Europe is a continent with many countries. It will depend on which other European country you are flying to. Some may be routed from Terminal 5 but it is highly unlikely that BA will surrender any slots out of Gatwick. If BA gives up slots at Gatwick they know Virgin will almost certainly grab them. Behind the scenes BA and Virgin hate each others guts, as numerous court cases have proved. BA isn't going to do anything that will help Virgin. It is much more likely that BA will move the last of it's short haul operations in Terminal 1 to Terminal 5.

Thanks for your info: too bad for me, I still must go from Heathrow after arrival from Vancouver to Gatwick to go to Pisa.

Leslie, which airline are you using? Some of the charters operating from Vancouver (Zoom, Air Transat) fly into Gatwick. Any chance you could change your flight?