Also the old F5 DNS/GTM "Dependency List" option is nice under the Virtual Server https://techdocs.f5.com/kb/en-us/products/big-ip-dns/manuals/product/bigip-dns-concepts-12-0-0/2.html#taskid 

Update GSLB pool configuration with Minimum Active Members to 3. This means the VIP will be marked down if fewer than 3 servers are up (i.e., if 2 or more fail) . The BIG-IP DNS monitors the VIP. If the VIP is down, GTM will failover to DC2.

here is an example 

#!/bin/bash
POOL="Your_Pool_name"
THRESHOLD=2

UP_COUNT=$(tmsh show ltm pool $POOL members | grep "Available Members" | awk '{print $4}')

echo "[$(date)] Pool $POOL has $UP_COUNT members up" >> /var/log/pool_monitor.log

if [ "$UP_COUNT" -lt "$THRESHOLD" ]; then
    echo "[$(date)] Pool DOWN: Only $UP_COUNT members up" >> /var/log/pool_monitor.log
    exit 1
else
    echo "[$(date)] Pool UP: $UP_COUNT members up" >> /var/log/pool_monitor.log
    echo "Pool is healthy" 
    exit 0
fi

Hello Brandon_TN​ 

You could use an External Monitor with a custom script to achieve you goal
check this https://my.f5.com/manage/s/article/K71282813

You could then create a dummy Virtual server with a dummy pool and assign the external monitor to it
And finally use this dummy VS as Dependency for you actual VS in GSLB