I installed Fedora 28. But I resized the partition that contained Fedora (/dev/sda7) wrongly using GParted and now I can't boot my system. (Note the partition format is ext4)
Resizing a partition does not implicitly resize any filesystem it might contain. You should have shrunk the filesystem and then shrunk the partition. (I'm surprised
gparted didn't warn you.)
However, to try and fix the damage, resize the partition back to whatever it was before. If you're not sure of the value then make sure it's at least the size it was before.
If you have already used the space then all bets are off, unfortunately. You might get some of your files back with a filesystem rescue tool. Or you might not.