As a lender, your action to recover the loan to follow banks approach.

If borrower cannot repay, bank will sell off the collateral to offset the loan.

In your case, sell the GPU to offset the loan since it is borrower intention to pay in kind.

If the sale proceed cannot cover the outstanding, then you decide whether to sue him or to write off.

If you still insist on cash repayment, then you have to give more time to the borrower or to sue him.