International check-ins usually close about an hour before departure. You simply don't have enough time, so as globaliser suggested 4 hours for comfort is a good idea.

However, as you're not through ticketed if anything goes wrong with your first flight the airlines are just going to wash their hands of any missed connections, it may seem a little extreme but have you considered taking a flight the previous evening and using a hotel for the night. If you're a worrier like myself, you might prefer the peace of mind. You ought to be able to priceline a gatwick hotel cheaply.