Jetway replacement should NOT be enabled for 3rd party airports so, any further discussion applies only to default airports.
Since SU10, the content.xml file doesn't control reordering of sceneries and should not be edited by hand, you must use the package re-order tool.
However, this is not even required, since the Package reorder tool works ONLY on the packages in the Community folder, this is basically a bug or, more precisely, a confusing feature of the tool since, regardless how you reorder, packages will always be loaded starting with the "Official" folder, then the "Community" and, within the Community folder, according the order set with the package reorder tool, otherwise it's alphabetical, if the tool is not used.
This means, since GSX replacement Jetway files are ONLY meant to be used to replace default jetways at default airports, which are all located in the Official folder, considering the GSX packages are located in the Community folder, they will always loaded after everything in the Official folder.
HOWEVER, if you have ANOTHER product in the Community that is also redefining airports, THAT one might getting an higher priority over the GSX jetway package, so you need to use the reordering tool to have the GSX package loaded last.