If the reason your gold will not precipitate from solution is caused by excess nitric acid in solution heating the solution with a button of gold will help, the nitric acid will be consumed when it oxidizes the gold, the gold oxidized gold will join with the chlorides from the HCl in solution.
You may or may not see the solution beginning to turn brown from gold beginning to be reduced, from your previous additions of sodium metabisulfite.
reaction with SMB:
Sodium metabisulfite reaction with water forms sulfur dioxide gas and sodium bisulfite,
Na2S2O5 + H2O --> SO2 (g) + NaHSO3
Gold chloride and sodium bisulfite and water forms solid brown gold metal powders, hydrochloric acid, and sodium bisulfate, along with evolution of a sulfur trioxide gas from the sodium bisulfate which converts to sodium sulfate in solution,
2AuCl3 + 3NaHSO3 +3H2O --> 2NaHSO3 +SO2 (g)
and
2NaHSO4 --> Na2SO4 + SO3 (g)
The SMB may have already reacted in solution as in the above equations, or in a similar fashion, so that it may not be able to precipitate the gold as it would normally, you may not have any sulfur dioxide gas and sodium bisulfite left in solution that it would be able to reduce the gold back to metal.
I would concentrate the solution with heat and the added gold button to consume any remaining nitric acid, after concentrated well but not so far as to form salt, dilute solution and let it sit (overnight), if gold precipitates, you will have a brown powder, test the solution with stannous chloride to see if all of the gold has precipitated, if not add more SMB, if no gold has precipitated you may see white powders (silver and possibly lead chlorides) decant the clear yellow solution and filter the clear liquid then proceed to precipitate the gold with fresh SMB (using your SnCl2 testing to determine the end point).
