Let me guess, when interesting traffic arrives at the Fortigate it is able to successfully start a new VPN tunnel and start passing traffic. However when interesting traffic arrives at the Check Point, IKE negotiations fail in Phase 2 and the traffic cannot pass. Fortigates are similar to Juniper/Sonicwall in that Phase 2 subnet/Proxy-ID proposals presented to it must match its configuration precisely, unlike Cisco and Check Point who will accept a subset of their subnet/Proxy-ID configuration in a Phase 2 proposal. You must adjust the Check Point configuration to present the exact subnet/Proxy-IDs that the Fortigate wants in Phase 2.
Read scenario 1 of this SK: sk108600: VPN Site-to-Site with 3rd party
And this SK for the proper filename of user*def file to edit: sk98239: Location of 'user.def' files on Security Management Server
You pretty much are stuck going down this road with Fortigate/Juniper/Sonicwall and to some degree Palo Alto interoperable VPNs.
Also as noted earlier make sure the Phase 1 and Phase 2 lifetimes match exactly, as Delete SA processing upon tunnel expiration does not always work correctly in an interoperable scenario and can cause tunnel hangs.
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