Inversion / confusion in binary packages
If I understand correctly, you kept the same structure as the Debian source package i3-wm.
You chose to name the metapackage i3-gaps and the "real" package i3-gaps-wm. So all the files debian/i3-gaps.* must be renamed debian/i3-gaps-wm.* You also have to update the contents of the files (like in *.doc-base, *.install)
In debian/control: The i3-gaps metapackage must not have dependencies on lib* and lib*-dev, libc6, gcc... In general, the lib*-dev are dependencies of -dev packages. And if a package requires a dependency on a lib*, with the ${shlibs:Depends}, the dependencies will be added automatically during the build. I suppose you had to add these dependencies because of lintian warnings, but when you will have rename all the debian/i3-gaps.* to debian/i3-gaps-wm.* the warnings / errors will disappear.
You must remove the "Conflicts: i3" from i3-gaps and add it to the i3-gaps-wm package.