What is stopping you from having a level headed, heart to heart conversation with your new colleague about your growing concern? Part of any job is to pass on the culture to new comers. I'm sure they'll understand.
Frankly, I'm not.
I think this conversation is actually the responsibility of the GP's boss, not the GP. If the GP is really confident in the existing culture, then in my opinion they should approach their boss with their concerns.
Be an adult and just talk to people. Explain why his behavior disrupts a nice culture you had and why this causes stress to you. If there is risk of heat and you are new to giving such feedback, the standard template is something like “I’ve noticed that you ..., which causes ... to me. Because …. Would you mind … instead.” Be sure to emphasis on your own feelings of the situation, don’t make it hostile or negative in any way.
This guy probably just comes from other team culture where people turn off notifications off hours.
Don’t bring your boss into this unless the pattern repeats. The message will be lost in translation and you will come of as a whiny wimp, both to your boss and your colleges.
Management usually doesn't want to discourage such behaviour because it fits perfectly into their all-too-common willingness to exploit such tendencies by throwing morsels of acknowledgement/praise to extract more work by feeding this hunger for visibility. It is rare that the work-life balance culture is widely consistent and respected by everyone including management and even if that were the case, any advise from management to tone it down generally results in such people sulking and continuing with their behaviour in more covert ways that gives rise to underhanded politics. All this is amplified if this kind of person doesn't want to leave - basically a fish polluting a pond personified.
My team is under no obligation to keep up with me, but Im not going to lower my performance for them.
I'm not going to pass judgement here, as it's a complicated issue and I have zero context, but I hope you understand why this is not an entirely truthful thing to say.
Working over your vacation sets a tone whether or not you intend it to.
I'm asking because I tend to dive into work headfirst. I love doing this stuff and I can't believe I'm getting paid for it; but if this is a thing perhaps I should be a little quiet about that - I'm not interested in making enemies of people by being 'too peppy'.
Yeah sure - most metrics in software do not work as expected, so the only thing that we are left is perceived performance. And perceived performance is created only by comparison - You always subconsciously look for the baseline observing people in Your team, so if one ot them is concentrated only on work and is at least a little bit competent while doing it, it makes others look worse in comparison.