gvn approve (nee ack) is for the OWNERS files access control; it
can be used for code review, but isn't really meant that way.
The plan would be to add changebranch support to Mondrian
(Google's internal code review app) and code.google.com.
As for how to see it? gvn log (pending in epg/gvn-log@950) would
show it, as would Mondrian, and as could any other gvn-supporting
review application. The information is stored in the gvn:change
and gvn:approved:* revision properties; see check_owners.py for
an example.