If I was you I'd just play the bat in first, by giving catches and having light throwdowns with a used good quality ball whilst playing defensive shots. I'd be weary of the GM now as my mate had an GM Epic LE and seam marks appeared on the face which suggests it needs more knocking in...so don't rely on the GM Now service - although generally most of the GM's seem prepared and don't show seam marks during the first stage of playing in.
Then when I've had throwdowns I normally go onto the bowling machine for a couple of hours playing half volleys, or you could set it up on a good length and challenge yourself. By now you should be seeing an improvement in the response from the bat...especially on of centre shots etc.
When I am happy with the bats performance against the machine and I am sure it is knocked in, I then take it to nets against live bowlers, and play normally instead of playing with a more defensive style...like you would with the throwdowns when checking if the bat is fully knocked in. If there is no seam marks appearing after this, I will then start bringing it into play during matches etc. By then the bat should be performing well but remember it will only get better, and with your bat being a GM they are generally quite hard so take a little longer to play in and get going