You need to add GOPATH/bin to PATH.
PATH="$GOPATH/bin:$PATH"
Update: Starting with Go 1.8, GOPATH defaults to $HOME/go
if not set. The above will not work if GOPATH is not explicitly set.
To set both, add this to your .profile:
export GOPATH="$HOME/go"
PATH="$GOPATH/bin:$PATH"