Morning oscar!,
I haven’t used Gluster in my OpenNebula setups so I can’t help you with that 
but as you mentioned that you want to use Gluster with OpenNebula 5.2, these recent conversations in the forum will provide you info about Gluster and OpenNebula 5:
I’m sure that users in the forum that are working with Gluster will help you with your setup.
For you second question, you can’t run VMs on an image datastore, as OpenNebula transfers images from the image datastore to the system datastore in the node where the hypervisor runs the VM.
Cheers!