My first idea would be to write them in italics. The "He signed," "He gesticulated," or "He motioned" seem fine, but it's a matter of personal preference and style.
Alternatively, as the viewpoint character meets this mute character, they could start out not quite used to this. Depict this with the description being clunky and complex, like their thoughts would be trying to figure out what this person is "saying."
Then he made a sign, and (maincharname) was afraid for a second that he didn't hear. "What?" He made the sign again, and with a flash of embarrassment (maincharname) realized that there was nothing to hear. His mouth didn't move at all, there was nothing to hear. He wasn't trying to get his attention, he was signing.
"Over... There?" He nodded.
As the story progresses, this could be simplified gradually until, symbolic of the character finally being accustomed to this form of communication, it's simply "He signed" or "He motioned."
He moved his hands (again), and [maincharname] realized he meant "It's just that way."
He moved his hands to sign "It's just that way."
He signed "It's just that way."
Sorry if I'm hard to understand, I kinda taught myself using janky methods and only recently (so, two years after I started writing three years ago) started reading books and stuff about writing. Hope this helps.