I`ve managed to make my GCE5003 multi region, but I`m not entirely sure how (and I can`t undo it to try again). But thought it would be best to post something for others to try.
Switch on the player with the infrared remote attached and no disk inserted.
Once it has started and displays no disk press the following
Title, Menu, 0, 0, 0, 0, 0
This should display a message which says "VERSION" - This is mentioned on the Roadstar 1437 hack. Also found that Title, Menu, 1, 2, 3, 4, 5 brings up a different VERSION message.
If the player is not yet multi region, you could try pressing enter, then 0 after the VERSION message.
Hope this is helpfull.