Why? If they're already using gulp and have people in-house that understand gulp, why is it so unreasonable that they don't want their consultants adding a new build system to the mix?
Well, I was implying that those projects in question are greenfield not inherited ones but even in the case of inherited projects, I think that devs/consultants have bargaining power when negotiating contracts to work out a compromise when it comes to non core details of the project like build systems and the like and project owners or managers could accommodate the consultants' requests regarding these points.
A lot of consultants work with existing teams. As someone who has worked on such projects from the existing-team side, it would be very off-putting for the consultants to come in and demand to change this sort of trivia before they've contributed any value. Having said that, if they come in and contribute a ton, and then point out that they believe they and the rest of the team could be more productive by switching to such-and-such tool, then they will probably be listened to. Consultants don't come in with credibility, they earn it.
Why? If they're already using gulp and have people in-house that understand gulp, why is it so unreasonable that they don't want their consultants adding a new build system to the mix?