Some of the modules are very noisy and will degrade and or completely jam the GPS signal if your not careful. This may not be the case for the processor you are using. The gadgeteer module comes with an external antenna. In general the further you get this from the CPU the CPU and anything connected to it e.g. power wires of IO lines.
The antenna needs to see the sky for the best performance. Modern GPS receivers can actually work with a pretty poor weak signal(even indoors), but they do this at the expense of dynamic(moving) performance. You might notice that position overshoots at corners and or the reported signal strength dropping when you change direction.
A plastic enclosure will not make too much difference, but it’s good practice to leave an air gap between the patch and the plastic so you do not detune it.
For best performance the patch should be placed on a large ground plane horizontal to the earth. The ground plane will also help shield it from any interference from below.