Virtual Desktop Infrastructure (VDI) and Virtual Machine (VM) environments, such as Citrix XenApp and XenDesktop or VMWare Horizon and Vcenter, support access natively through HTML5. You can RDP, VNC, or SSH to these machines through Clientless VPN without requiring additional third-party middleware.
In environments that do not include native support for HTML5 or other web application technologies supported by Clientless VPN, you can use third-party vendors, such as Thinfinity, to RDP through Clientless VPN. Reference: https://docs.paloaltonetworks.com/globalprotect/10-1/globalprotect-admin/globalprotect- clientless-vpn/supported-technologies
https://networkwiki.blogspot.com/2017/03/palo-alto-networks-clientless-vpn-and.html