Whether this would or would not have been legal now in Washington, it wasn't then. They got caught. Gary was charged along with Rebecca, in his case as one who "make(s) available" his house for producing a controlled substance.
Gary came up with an ingenious argument, or so he thought: Gary's meretricious or committed intimate relationship with Rebecca had made her a co-owner of the house. Because of that, she didn't need his permission for her indoor gardening, and so he hadn't, under the governing statute, made the property available to her for illegal purposes. His co-owner was acting on her own (OK, at least after he did the construction work).
The trial court rejected his proposed jury instruction to this effect, and the Court of Appeals found similarly in an unpublished opinion: "Regardless of of (Rebecca's) ownership status, the law does not permit (Gary) to knowingly make the property available for her unlawful use."
So Gary's conviction stood. And he had argued on the record that Rebecca shared ownership of the house. This latter might prove more costly to Gary than his one day of confinement, with credit for time served.