Is there a way to get the resolution of the users monitor in game?

Guis will automatically scale to the client's moniter if you're using the Scale member of a UDim2, rather than the offset.

UDim2s are setup like this;

{Scale X, Offset X}, {Scale Y, Offset Y}

So, you're going to have to set the Position and Size properties on the scale sides, rather than offset. 1 being the whole screen, and 0 being none of the screen. Think of it like a percent, but from 0 to 1. So, naturally, .5 would be half of the screen.