Hum, nope?!?
GTM can add 3rd part servers on it.
Doing this, you'll lose LTM metrics from servers, because LTM is not there :)
But you can run GTM with other server and monitors.
For example, I can work with GTM and a generic FW that lays all of my HTTP services, etc.
I don't need to move FW behind LTM nor change all my infra. Because DNS is just a name resolver and I don't need to traverse it as well.
But it make sense to monitor the generic host virtual server to give me availabity or disable "Verify Member Availability" on pool to ignore available status.
Make it sense?
Regards.