Can I do a giveaway here and ask people if they could subscribe to my youtube channel optionally?

but he can't win if he don't has the ga code, so I give it to him only if he subs
(I don't even have a youtube channel, so this is just a hypothetic question)

Not directly on the forums, no. If that was permitted, the forums would be flooded- there's already the occasional new (and quickly suspended) account trying that sort of thing, as is. On the other hand, you can freely make a "Hey, promoting my youtube channel with some giveaways!" thread, drop some giveaways in it, and put the links to your youtube channel within those giveaways. Staff'll take action if someone is posting fake giveaways or otherwise being exploitative- but otherwise, the only difference between that and any other giveaway thread is the associated narrative [At least, assuming you're not breaking any other site rules, such as linking to content related to copyright infringement or pornography, or making use of referral links].
The main thing is that advertisements shouldn't form the primary basis of a thread, as outside sites should only be referenced directly on the forums if they have a direct relationship to the themes of SG (ie, a site offering a game deal). If giveaways (which inherently have a direct relationship to the themes of SG) are the basis for the thread, then really, who is going to nitpick at the details of a few out-of-the-way comments?

In short, the intent is to avoid spam and exploitative use of the site, not really to prohibit reference of your personal sites altogether.
As an example, somewhat recently a particular storefront had their promotion thread closed, because it wasn't related to any particular time-sensitive offer, and thus just cluttered up the forum with perpetual advertising. If rules weren't in place to prevent such, we'd have a front page covered in youtube and storefront threads, with little else being able to stand out in the flood. By putting in a "claim yourself a spot on the forums with giveaways" exception, the site both sets a standard under which exploitative threads can be filtered out, and encourages support of the site's core theme.

tl;dr version: You can advertise within giveaways, just not on the forum.

Do you know of such a website?

Oh, I know about those (still, thanks for linking!), I hoped there was a thread revealing how many people actually subscribed to a YT channel linked in a public ga.