Getting to Heathrow from Gatwick and timelines

1) Don't do this. Too risky and too much stress.
2) If you absolutely insist on doing this:

• It is not necessary to be at Heathrow 3 hours before an international flight, just for intercontinental flights. For European flights 2 hours should be enough. Some airlines have a hard cut-off at 1 hour. You will need to check the airline's website to find out.
• You haven't allowed any time to de-plane and get through immigration 1 hour to 90 minutes would be typical.
  • Some gates at Gatwick are 10-15 minutes walk from the terminal
• You haven't allowed for the incoming flight being late. depending on wind, transatlantic flights can be +/- 1 hour.
• "will be hiring a car to get to Heathrow" - I hope you mean pre-booking a taxi. Hiring (renting) a car to drive yourselves is not a good idea.
• The drive from Gatwick to Heathrow can take 1 hour on a good day 2+ on a bad day.

11:20 + 1 hour late flight contingency + 1½ hours immigration and walk through airport + 2 hours by car = 15:50
Just about legal for a 17:30 flight. But not conclusive to a relaxed trip.

Doing this on separate tickets makes for a high stress situation.

If you cannot get Canada to Oslo on a single ticket, it may be a good idea to spend a night in London. Takes the stress out of ( possibly) missing a plane and helps with the jet lag, so when you arrive in Oslo you all are more 'ready to go'.

If you cannot find an airline that will sell a single ticket for the trip, take that as an indication of what the 'experts' think of doing what you are trying to do.

I frequently fly into London (usually Gatwick) and then want to fly on a separate ticket some where else. I ALWAYS spend at least one night in London. As mentioned above, too many things can go wrong and if you miss that flight to Oslo you will need to buy an on the spot ticket (very expensive) and there might not even be three tickets (you and your parents) available on the same flight the next day. You could be stuck with very expensive tickets and last minute hotel rooms, possibly for a couple of days. Is it worth it? Enjoy a day or two in London and have a much less stressful trip.

My reading of the Norwegian airways timetable is that they have a flight which leaves Gatwick at 18.55 arriving Oslo 22.00 (there is a one hour time difference, the flight itself is just over 2 hours). This is a late hour to arrive in Oslo, but would remove the uncertainty over the journey between the two airports. Have you rejected that option?

So we just booked our flights and we get in to Gatwick at 1030 am actually and we could fly out of Heathrow to Oslo on British Airways at 1925. Does this seem more reasonable? We are going to spend a couple of nights in London on the way home and would rather just get to Norway.

DIY:

https://www.heathrow.com/transport-and-directions/travel-between-airports

https://www.gatwickairport.com/to-and-from/gatwick-to-london/to-other-airports/

You are way over the connection time an airline would risk on a single ticket, but it is you taking the risk.