You simply have to pull out the rubber seal to make a gap then
I really wouldnt try that. Most modern windows have the gasket co-extruded onto the UPVc for a permanent bond. It might be an old school gasket that fits in a channel but probably not. Tear it of and it won’t go back on. Also bear in mind windows are supposed to secure, if it was easy to remove the gasket and access the locking mech they’d be pretty useless.
Jambourgie has the best suggestion, get the handle off and check the spindle.
Funnily enough that’s the second time today I’ve been told I can’t lick windows anymore, what gives?