How do I mount a qcow2 disk image?
A quick google search turns up the qemu-nbd program, mentioned here. It is part of the qemu-kvm package, so you'll have to install KVM if you aren't using that already. Not sure about any direct GNOME/KDE solutions, if that is what you were looking for. Here is an example for using it:
sudo modprobe nbd
sudo qemu-nbd -c /dev/nbd0 --read-only /path/to/image.qcow2
udisksctl mount -b /dev/nbd0p1
There's also libguestfs, but it's not yet available from official repositories1. There are binaries in libguestfs.org though.