I think that doing giveaways on gaming channels to get new subscribers is rather lazy and cheap.
Like, I earned my 102 subscribers fair and square - no giveaways or anything. Granted, on my theory videos I asked the artists if I could use their art and they all wanted links to the video, so that could help with view and subscriber count, but that's why I try to keep said theory videos to a minimum.
If someone says they're gonna give away a $50 or $100 gift card for ANY sort of online store or shop (amazon, EB, Xbox) or a code, but you have to subscribe to their channel to be entered into it, that's kind of a cheap way to get subscribers. And what's worse is that they never have a picture of said card to prove they're being legit so you never know if there really is a giveaway.
Like, c'mon, just practically buying your subscribers is the worst way to build up your YouTube channel.
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Eh.. Doesn't sound too unethical to me. If they're able to expand their "business" or whatever you'd call it then I'd say it's smart. I won't be subscribing to those sorts of channels regardless.