plan to put a gateway server in a untrusted domain/forest to manage about 100 servers over there. what's the reasonable hardware config for it? mainly CPU, RAM and disk space. The gateway server will be built on server 2012 R2.

thanks in advance

Hi,

I suggest you use System Center 2012 - Operations Manager Sizing Helper to determine the hardware requirement.

http://www.microsoft.com/en-us/download/details.aspx?id=29270