If your gold is already auctioned try to find out at what rate it was auctioned and when the auction was conducted. Find out the realized money after auction minus the principal amount + interest due from your side. If there is any leftover money subsequent to auction after deducting the principal amount + interest, demand from them that the money recovered from auction over and above the money due from you, is to be paid to you.

On the other hand if there is any shortfall in the proceeds of auction money to set off the loan and if they are pestering you to pay interest take the stand and inform them that gold was pledged for a lower amount than the market rate and the matter should end there, if it does not end, issue a legal notice through a lawyer stating that since you were unable to service the interest and had informed them time and again about your inability and had requested them to auction the gold. However a look at agreement you would have entered with them at the time of pledging gold has to be seen before coming to conclusion.