Hi Scott,
I don't think the replacement nibs from Craft Supplies fit the Gent or Jr. Gent pens. And I don't know of anyplace out there that has an easy replacement.
The customer's interest in a platinum and a 10K gold nib is interesting. Neither of those is what you would call "usual" in the fountain pen industry. If it's a gold nib, it's usually a 14K gold, or even better an 18K gold. I have a Parker Sonnet with an 18K nib that is a sweet pen! I don't even hear of Platinum nibs. Maybe I run in too small a circle! ;-)
What we think of as the "nib" is usually the nib and nib holder assembly. The nib is actually the metal piece at the end. There are pen repair places that would re-work the nib into a broad point, or would replace the nib completely with a gold nib of whatever width you wish. They will also charge a decent amount to do it. So far I haven't found it worthwhile to do it, but I can tell it's coming at some point! Just do a Google search for Fountin Pen Repair.
Scott.