GSX can not remove SODE jetways!
You have to do that yourself. Normally the SODE-files are stored in C:\ProgramData\12bPilot\SODE folder (Yours may be different) and then open the xml folder.
In the xml-folder you find the file for Nantes.
Backup and then delete this file.
Now your SODE-gates at Nantes are gone, and you can put GSX-jetways.
This is a very short explanation, there are more topics in this forum that describe your problem more in detail.
Go look in the forum, it may help you better.