I believe you meant to say SRX340 instead of SRX345. Yes, on the SRX340 ICMP probes for RPM is not supported.
Instead of monitoring the next-hop (gw) IP with icmp I would suggest to monitor some service on the internet, for eg:http://www.google.com. This would be an HTTP GET test which is supported. This way it would also ensure the internet path is validated rather than just the next-hop reachability.