Hi Raistlin,
How much bandwidth on average does a Tor gateway eat up in a month?
You can set the maximum amount of bandwidth you'd like it to use in the config file.
And will that Tor gateway still remain anonymous even if I run other 'Public' services on the same server?
The tor relay isn't anonymous - its IP address will be widely shared. It's used to keep tor users anonymous. Unless you're referring to hidden services, in which case the answer is 'yes', I think, as long as you're careful not to leak anything between the hidden service and the public ones.
Nick