A Busuu example dialogue suggests that this phrase ギターが弾けると良いのですが means "It's better if you can play the guitar". I understand that ギターが弾けると means "if you can play the guitar". Is this the explanatory の in this phrase? What purpose does it serve? Does it make 良い mean "better" rather than just "good"? Also, is this a typical way to express that "something is better/preferred" in familiar language? I would have just said ギターが弾けるの方がいいです...
「ギターが弾けると良いのですが」means "It's better if you can play the guitar".
I am not sure if the translation is completely correct or not. I'd like to know in which context the phrase is used. i.e. Are you asking if I could play guitar or not at the venue? Are you asking there is any guitar instrument other than instruments? Or you may want to play guitar better than now?
Also, is this a typical way to express that "something is better/preferred" in familiar language?
It depends on how you ask though, I think it can indicate one's preference. The examples I prepared are
- ラーメンを食べられると良いのですが。: I'd like to have miso-ramen.
- 外の景色が見れる席に移れると良いのですが。: May I move to the table you can look out the view?
- Wi-Fiを使えると良いのですけれど。: I wish I could use Wi-Fi. (Is it possible to use Wi-Fi)
If you want to use in a more comparative way, 「ギターを弾けた方が良いです。」is correct. I think「の」is used to express comparing concrete objects i.e "Being able to play guitar" is more conceptual.
There are several grammar points in this sentence.
The A と B here means that if you do A, B happens automatically. For instance:
If you press the button, the ticket comes out (automatically).
The の is making the sentence stronger. It is used as a strong explanation ("but") or a strong confirmation ("really"). In spoken language this is often just ~んです. For instance, いいんです、いいんですよ (while the yo is an even stronger confirmation of the sentence). In written languages or formal Japanese ん becomes の but has the same meaning.
Since there is an が in the end of the sentence it seems to be an answer, which would serve as an explanation: "but".
So this sentence means (probably) someone asked a question and the answer is:
"But being able to play the guitar, is really good"
Using the comparative form in English translation doesn't mean that it's the same in Japanese. In your sentence I wouldn't consider 良い to necessarily mean "better". You could as well translate it as "It would be good if you can play the guitar".
Also your "ギターが弾けるの方がいいです" is grammatically wrong, you'd have to say "ギターが弾けた方がいいです" or "ギターが弾ける方がいいです" (somehow the latter doesn't feel quite right, though…)