I’ll echo these comments and toss in the math.
Let’s say you gather up 50 forum folks for beta testing. Yes, a few will “mooch” and read your almost-done WiP and not provide any useful feedback but most will provide helpful feedback that at least justifies their free reading. MANY of them will go above and beyond, not only pointing out issues but suggesting possible solutions. And a few will just dazzle you with insightful analysis that will greatly strengthen your work.
At the very least, these people found typos, spacing errors and continuity errors, but they likely provided far more feedback.
Even if none of them buy your game (worst case scenario!) you lost a total of…wait for it…*$17.41 in sales. $17!!! And of course as @Havenstone points out, many of them WILL buy your game for the reasons explained above.
Doesn’t all the beta testing seem like a bargain now??? You could barely buy 2 hours of beta testing conducted by one solitary minimum wage earner (in the US) for $17!
That $17 in lost sales (again worst case scenario) “bought” you a ton of help that should result in better reviews on the platforms, better word of mouth, improved sales, and maybe even customers who will consider buying your next gamebook.
*(calculated assuming a $1.99 HG release x author share (.175) x 50 lost sales)