Assuming your Westjet flight arrives on time (which cannot, of course be 100% guaranteed) you should allow at least
an hour from arriving at the gate to a) deplane b) get to and through UK border control c) collect your bags d) go through UK Customs and be landside so you can bag-drop for your Easyjet flight (I assume you'll be checking-in online).
Fortunately, both Westjet and Easyjet normally use Gatwick North Terminal
. Changing to Gatwick South Terminal
would take an extra few minutes.
Easyjet close bag drop exactly
40 minutes before departure time. In your case that is 1215. When they say 'exactly', they mean it. If you're a minute or so late you'll miss your flight with no refund. Bag-drop opens 2 hours before departure and, depending on the flight, there can be long queues. I suggest you join them as soon as you can.
Make absolutely certain that your bags (including cabin bag/purse etc) are within Easyjet's size and weight limits. They do
check and they will
charge you extra if you are overweight or over-sized.http://www.easyjet.com/en/help/baggage/cabin-bag-and-hold-luggage
3h 15min from your Westjet landing/gate time to Easyjet check-in/bag-drop closure is plenty of time assuming all goes well. It even gives you a little leeway for inbound delays but do be aware that if you are badly held up (weather and technical issues are unforseeable) and are late at bag-drop or gate, you'll miss the flight with no refund.
I'd strongly advise you to be through security and airside at least an hour before your Easyjet flight departs. Security queues can be lengthy and gates can be some distance away.