Not sure it belongs to gedit, but I think we shouldn't only rely on the HTTP header "Content-type" while opening a file from a remote HTTP server. This is problematic because code files are often sent as text/plain, no matter what their actual mime-type is. (and some servers interpret files being declared as python or php or perl, so sending them with right mime-type might be difficult). This causes gedit usually not to highlight the file when opened from HTTP (from a svn server or so)

For instance, try opening
https://svn.tinymail.org/svn/tinymail/trunk/libtinymail/tny-list-iface.h
and notice there is no syntax highlighting.