He didn’t forge documents. He just didn’t want to tell the protagonist that he was the heir. I believe the letter also told him not to tell. So, in a way, he was fulfilling the dead mother’s wishes on that.
Gustav’s was failing. Selling cheap food for profit might have been the only way to keep the business afloat. Yes, he tarnishes the name, but sometimes things have to be done, and he might have had to make that hard decision.