I don't know if this is possible as a VPN is to join internal lans???
I'm curious how you get on with this.
How about a rule on the gateways the traffic traverses:
SRC:Guest-networks, Dst: 'Negated' Internal-networks, port: http/s etc
or dst: all_internet.
Basically all networks except internal. Routing and Anti-spoofing considered.
If you still want to go the VPN route then I would test with one tunnel per GW pair in tunnel settings.
So the GW will consider the Encryption domain during authentication to be IPv4 Universal Range, i.e. as long as both sides are configured the same the auth will be happy with any IP in the Traffic Selector - Initiator and Traffic Selector - Responder but the traffic will only hit the community if there is traffic that consists of the Guest-networks and 'other'.
On the External GW the Encryption domains would be Guest-NW object for the peer, but for the local I see as the problem.
I don't see the objects you can specify like all_internet or all networks regarding Enr Doms.
You could create an NW object with 0.0.0.0/0.0.0.0 and see if you can use that specifying a specific VPN domain for the GW community in GW Properties. Or leave as is and test because the initiating traffic will be coming from the internal FW so auth may make it through.
There is the radical option to change and test traditional mode where you have to specify the VPN community in the VPN column of the rule. - Guessing that is not recommended by CP now though.
There is a warning that config will applied to new policies only so this would be a headache.