The gaskets are for the top filling part. If you rotate the top of your tank it swings out and you'll see one already in there. Without the gasket it'll leak out and the thing will fling out at any time, with the gasket it actually works quite well. However if you leave it in your pocket or something, you might want to toss one of the included vape bands around it to prevent it from accidentally opening (happened to me twice before I started using the vape band).
Another little note, if you have a larger tip on your bottle of juice you can remove that gasket for an easier time filling then just pop it back in, the metal opening is much larger than that of the rubber gasket.