And there, you said it all. Without saying much, really. Yes. G is the same word which was the bone of contention in Honey Singh's Party All Night in Akshay's Boss. It's.. well, some adult supervision may be needed here, gaand, which means arse in older English, ass in modern one, and bum in a rather Khoobsurat language.
However, the word is just uttered as 'G' in the song, and one can simply consider it Hindi 'Jee' which means heart, and you could even say you have to dance Jee Phaad Ke, that is, opening your heart, with all your heart, freely, if you will.
However, the makers probably didn't want this to be taken in a wrong way, and hence mentioned the song title as 'G Phaad ke', so you know it's not the heart, but another part of your body literally behind you.
And, oh, if I missed the meaning of G Phaad ke in the whole drama, it would still mean 'without any inhibition' here, the same if you thought it was 'jee'.