what's the difference between gateway_icmp and icmp in the BIG-IP monitors?

The gateway_icmp monitor is pretty much a template that's intended to be used with "transparent mode" monitor configuration, where you should tune it to ping a specific IP address.

If you don't specify transparent mode, and just use it as-is with the default template configuration, then it will still refer your LTM object IP address, essentially working the same as the icmp monitor. 

For LTM pool objects, since every pool member is a combination of an <IP address : port> socket, the use of icmp monitor was disallowed since it won't be a reliable tool for checking service status, as it's just probing the host where the service runs on; while gateway_icmp is allowed as it makes sense that there might be dependancies on specific IP addresses (where the monitor is configured in transparent mode, checking different hosts).