Hello,
Short answer - mGRE with NHRP is not supported in JUNOS.
Long answer - the closest JUNOS feature is called "dynamic tunnels" https://www.juniper.net/documentation/en_US/junos/topics/usage-guidelines/services-configuring-dynamic-tunnels.html - BUT
1/ it is not supported on SRX series
2/ tunnels are established upon receiving a valid VPN route from distant tunnel endpoint
When a router receives a VPN route that resolves over a BGP next hop that does not have an MPLS path,
a GRE tunnel can be created dynamically
This could be a L3 VPN route or L2 VPN route.
If I were You, I'd look into another JUNOS feature that is supported on SRX and does not involve GRE - it is called AutoVPN
It supports OSPF without GRE and AFAIK, it supports CSCO spoke endpoints.
HTH
Thx
Alex