It's difficult to know what's going wrong. What effect are you seeing from that call, and what would you expect to see?
The viewport defines the area of the screen that maps to screen space quad with x,y in [-1,1]. It's essentially just a scaling and doesn't prevent rendering outside of that area.
If you want to prevent all rendering to the 100 pixel border, you should probably use scissor as well. You'll need to do this after every call to gd.beginFrame().
You might find it useful to read up about viewport and scissor on the webgl / opengl reference pages. The GraphicsDevice calls correspond exactly to these.