Make `gdk open` work on Gitpod

Gitpod doesn't have `xdg-open` command, because if doesn't run in
desktop environment.

Use Gitpods native helper to open local URLs in a new tab.

lib/gdk/command/open.rb

 # frozen_string_literal: true
+require 'mkmf'
 
 module GDK
   module Command
@@ -48,8 +49,10 @@ def open_exec
       def open_command
         @open_command ||= if GDK::Machine.wsl?
                             'pwsh.exe -Command Start-Process'
-                          elsif GDK::Machine.linux?
+                          elsif find_executable('xdg-open')
                             'xdg-open'
+                          elsif find_executable('gp')
+                            'gp preview --external'
                           else
                             'open'
                           end