At my own table, a Monk of the Silver Fist (MoSF) wearing a monk's robe would be treated as being able to make unarmed strikes as if a normal Monk of 5th level (i.e. 1d6 damage), with no change to their gauntlet damage. I would also allow them to gain the AC bonus of a monk 5 levels higher (since the MoSF still has this ability).
Note, however, this is likely open to specific GM interpretation (as another answer here shows), due to the now-ambiguous wording of the item. The Monk's Robe is from the Core rulebook, and when written, there was no such thing as a Monk without the core abilities. Thanks to archetypes, that assumption has not held true. If the item were written today, I expect it would refer to those specific abilities as being improved, rather than the class.