My first go at this I got a C- for sending mixed/inconsistent signals to Gary by at first agreeing to his request, but then saying no to a follow-up request.

I wish there was a bit more context about the overall status of the project - yes it is a risk to ask a dev to context switch, but it is also a risk to deny a trivial request from Gary. If Gary has a meeting with the client later and it goes poorly, that could be in part because of the lack of flexibility of the team to meet Gary's needs. If Gary is a bad leader, he's going to be doing a lot of fly-bye requests. If he is a good leader, he'll do it when it is truly necessary. In my view, writing and running a SQL query should be a quick an easy task even for a Jr Dev. At the end of a sprint, there should be plenty of things to demo even if search doesn't make it into this sprint. Also, I'm a firm believer in natural consequences - if Gary can be made aware of the consequences of bypassing the tech lead, he learns the hard way that it needs to be worth it.

I hear you! A good leader will do it only when it is truly necessary but we never know this ( atleast until we really know someone! ) Probably adding more context on Gary's nature of ask will help. Also, as mentioned in other replies, the point here is to provide heuristics and not a playbook.