A very slight "improvement" on the current answer, as there are less values to pass and in my test that solution resulted in a lower case z. Each is a work around due to limitations with the timestamp returned by Now().
Using NOW returns Local server time vs GMT so you will always have to adjust to local time. Assuming system time remains -6 (perhaps always or until there are global servers), you can use this to get what you want. It would be great if Now() allowed a parameter to return GMT but currently that isn't available. Additionally, if you are looking for a standard format, culture code is not needed, that parameter is primarily useful to display culture specific output, vs standardize output as is requested here. I have no answer for why a space is added between date and time. A format of "sortable" is also available for both date and time but is imperfect as well for your end goal.
Concat(Substring(FormatDate(DateAdd(NOW(), "6", "H"), "iso"), 1, 19), "Z")