libqt5gui5 is already depending on the needed X libraries, so
unless your app is doing something tricky, you don't need to care
about this. But we can tell more if we see the application in
question...
I am
packaging a gui that has dependencies for qt and such. How
do I go about ensuring that X is available as well? Do I
list that as a dependency as well. The upstream maintainers
don’t call it out specifically but it is understood. Links
to docs are always welcome.