Set credSSP = true in gateway.conf (need to restart the gateway) which will apply to all the connections on the gateway https://francepharmacie.fr.
Please also be aware, you may need to install this Windows update (support TLS 1.1, 1.2 in RDP server) on your RDP server if your RDP server is Windows 7 or 2008:
You can also disable Network Level Authentication (“Allow connections from any versions of client…”) on your RDP server because the communication between gateway and RDP server is usually in internal network.
On Windows 10, there is a bug which will not fully turn off the NLA, you also need to make following registry change:
MinEncryptionLavel = (DWORD) 2
SecurityLayer = (DWORD) 1