The word yog means the effort of joining one's mind with God, and a yogi is the one who practices that, practices doing that. So in fact, a yogi is synonymous with a devotee. Yog is synonymous with bhakti.
So this, in gist, is how Gita tells us to meditate. Notice that Shri Krishna did not say, “Meditate by breathing and just concentrate on your breath coming in and out.” That's a nice way to meditate, very relaxing.
But keep in mind, whatever your mind is dwelling upon, you get the qualities of that. So Krishna is Divine bliss and perfect goodness. So if you want to imbibe blissfulness, if you want to receive Krishna's grace, if you want to purify your mind, then you have to attach your mind to Him.
One shouldn't think that all meditation is meditation; it's all the same, and do any kind of meditation. Oh, if you're meditating on your breath, you'll get a nice relaxation, but you're not purifying your mind.
