<input type="submit" name="submit" value="Create" id="submit" />
<span class="or-cancel">or <a href="">Cancel</a></span>
Notice the empty href attribute. This happens if GP_URL_BASE is '/' and the current permalink structure has no trailing slash.
This is because gp_url() is the wrong function to use here, it only returns a relative portion of the url and what we really want is the GP root URL in this case.
We should use gp_url_public_root() instead.