Trigger GTM failover based on LTM available pool members

I am not sure if this would work but try doing this:

For an LTM-1 with VIP-1 sending traffic to POOL-1, create a priority group with 10 and place all pool members in that priority group. Create another priority group with 5 and place a dummy pool member that is marked down and set a priority group failover as less than "X".

Also, check out the post by Kevin here and this blog.