iOS/Android clients use only Basic authentication against Windows 2012 RD Gateway (HTTP transport), while it provides more secure options like NTLM. Both Windows and Mac clients use NTLM in that case.

Hi,

It seems that it used Basic authentication method for MRD client. For NTLM authentication need to verify the RDP security level setting in the remote PC options is set to Client Compatible. Please check below article for information.
https://technet.microsoft.com/en-us/library/dn473007.aspx

Hope it helps!

Thanks.

RDP security level is not related to RD Gateway authentication, which is happening on HTTP layer.

Hi,

For this you can specify the policy setting for RD Gateway authentication mode under below path.
User Configuration\Policies\Administrative Templates\Windows Components\Remote Desktop Services\RD Gateway

In addition you can refer beneath article if find helpful in your case.
Remote Desktop Gateway client fails authentication with Your user account is not authorized to access the RD Gateway
http://blogs.technet.com/b/networking/archive/2010/01/14/remote-desktop-gateway-client-fails-authentication-with-your-user-account-is-not-authorized-to-access-the-rd-gateway.aspx

Hope it helps!

Thanks.