If Harry is not the heir of Godric Gryffindor as per this question here, then who is in among all the other characters?
Per Was Harry Potter literally descended from Godric Gryffindor? -- as you note in this question -- Harry himself is not a direct descendant from Gryffindor, despite popular fan belief.
There is no mention of Godric Gryffindor's family or heirs in any of the seven Potter books. I personally know of no interview, online or otherwise, with J.K. Rowling where any of Gryffindor's heirs are discussed. Pottermore has new canon material available through Goblet of Fire, but I'm still reading that section of the site, so I don't know if Gryffindor's line is discussed at that point in Pottermore. I don't recall Gryffindor being discussed on Pottermore in the sections of Pottermore that review books 1-3. At this time, no relatives, family, or true heir(s) of Godric Gryffindor are known.
As always, anybody is free to correct me if there's some obscure reference out there!