After obtaining a virtual machine in the form of an OVA file, we need to expand the archive and convert the vmdk to a qcow2 format as specified during the GNS3 Import Appliance dialogue. The full instructions to convert an OVA to a qcow2 format can be found at cloud.garr.it.
tar xvf pa-vm-esxi-8.0.0.ova
A prerequisite package must be installed to provide us with the means to make the conversion.
apt -y install qemu
With the qemu package installed, we can begin the conversion.
qemu-img convert -f vmdk $FILE-disk1.vmdk -O qcow2 $FILE.qcow2