It is a bit colloquial, and there is dialect involved: "gurning" is pulling a strange face, often referring to silly competitions where old men take out their false teeth and do incredibly ugly things with their features, but in this case referring to the O-face that presumably occurs at some point while you're watching porn. I'm not sure why stick-up gets mentioned twice, but I'm thinking one occurrence refers to robbery, and the other refers to another side-effect of watching porn.
The app snaps a pic of you as you gurn, then sticks you up, and there's a bit of a flap on about that.