Yes, 50cm is close, but you mention Bruce Gilden's latest work. I think he photographs quite close, less than 50cm (even with a 50mm lens).
Yeah, I know I need to look at the guide number. And I know how to use a manual flash.
What I'm looking to understand is: if I get a TTL flash compatible with my M6, will it accommodate for all the situations I encounter, which include taking photos at night up-close (50cm) and from afar (10m).
If a TTL flash does not fully cover these types of situations, then I don't want to get one.