I think the ATF may have beat them to it although they aren't using the term 'gunwalker' as a noun.
The Department of Justice’s Operation Fast and Furious: Accounts of (four) ATF Agents
In that House Oversight Committee Report four ATF agents repeatedly use the terms 'gunwalking' as verbs to describe what they were doing. There is even a chapter titled "V. Gunwalking Defined: It’s Semantics"
They talked about *gunwalking* and that is why David and Mike renamed the op Gunwalker for their expose`.