the G-Man's briefcase is the same briefcase as in Pulp Fiction

Well!! It makes sense! In Pulp Fiction, the whole plot of the story was driven by the need for the contents of some mysterious briefcase, and we never see what's inside it except for an ominous glow. In Half-Life, the G-Man always carries around a mysterious briefcase, and we never know what's inside it. Maybe the characters of Pulp Fiction stole it from the G-Man for their own nefarious purposes. Maybe the G-Man had hired them (just as he hired Gordon Freeman) to return it to him. Maybe they were stealing it from the G-Man! Maybe they had absolutely no idea what was inside and they just wanted power. How many delayed games does it take to get to the Pulpy center of a briefcase? The world may never know.

Does this mean that Marcellus Wallus is the G-Man, or that he answers to the G-Man?

Depends on whether or not you think the G-Man "looks like a bitch."