hi,
Please go on troubleshooting it via the following steps:
1. From extranet environment, use Remote Desktop Connection to access the RD Session Host directly. Please note to add the Gateway server name into the following location:
Remote Desktop Connection – Options – Advanced tab – Connection from anywhere – Settings – Use these RD Gateway server settings <input the FQDN name of RD Gateway>
If it succeeds, it means there is something wrong in configuring RemoteApp for extranet environment. If it failed, go to Step 2.
2. Check the firewall configuration or anti-virus software on the server side. Especially, the firewall for Public Network or other Internet accessing network may block the Port 3389. If possible, please temporarily disable the firewall for all networks and test the issue again.
Hope will helps!!!!!!!
Thanks,