GCs really let you trivially not care about a LOT of things... Until you need to care. Things like where and when you allocate, the memory complexity of a function call, etc. With games you need to care about all that stuff so much sooner.
Once you're taking the time to count/track allocations anyway you might as well just do it manually. It just codifies something your thinking about anyway.
People understand seconds, and any other measurement would require specifying a lot of computer-specific stuff, and if you're going to do that, then you might as well fully specify the workload, too, to answer those questions before they come.
It isn't meant to be a precise answer, it's meant to put the GC performance broadly in context.
Languages not used for anything anyone cares much about don't attract much bad press. There are reasons why games are written the way they are. It is not masochism.
Edit: specifics from https://blog.golang.org/ismmkeynote - Go hugely improved GC latency from 300ms before version 1.5, down to 30ms, then again down to well under 1ms but usually 100-200us for the stop the world pause. So I guess it is stop the world but the pauses seem ridiculously small to me. They guarantee less than 500 microseconds "or report a but", which seems more than fast enough for game framerates (16ms for 60Hz frames). Am I missing something?