For IOS and platforms that support gatekeeper functionality you can use the Feature Navigator:
http://tools.cisco.com/ITDIT/CFN/jsp/index.jsp
Search for feature "gatekeeper" and then you can start drilling into platforms and IOS releases. Or search by Technology>Voice>Gateway Protocols.
You could also use this data sheet: http://www.cisco.com/en/US/partner/prod/collateral/voicesw/ps6788/vcallcon/ps4139/data_sheet_c78_561921.html
Or, read up on the various materials provided by Cisco in the product page for gatekeeper technologies: http://www.cisco.com/en/US/partner/products/sw/voicesw/ps4139/index.html
HTH.
Regards,
Bill