Was there a political body like the Gerousia for ancient Macedon? If yes, what was its relationship with the monarchy?
No, nothing that formal.
Gerousia vs. Hetairoi
That said, the king had "Companions" or Hetairoi. These were aristocratic landowners who were literal companions of the king; they fought with him, rode with him, hunted with him, and dined with him. They weren't kings, but could be king-makers, and he often tested out new ideas with them first, or turned to them as advisors in war and peace.
But unlike the Gerousia of Sparta, the king also MADE them. Literally. He could appoint men as Hetairoi. He could also remove the honor (as seems to have happened to Airopos of Lynkestis). Also, the king's Hetairoi didn't have to be Macedonian, or even Greek. While we know for sure only that Alexander named Persians, almost certainly the first Alexander made a Companion of Bubares, the Persian who married his sister!
We don't know their numbers, which surely expanded and contracted over time. Gene Borza estimates about 100 under Philip, and this may be right. Alexander's famous "Tent of 100 Couches" that he inherited from Daddy is a clue. So is the size of the first row (the only stone row) of seats at the Aigai/Aegae theatre. Gene measured it, and you could fit about 100 butts on it. Ha. Under Alexander, that number expanded a fair bit.
Last, what isn't clear to me is whether only the father was a Hetairos, or if that extended to his sons even while he was alive. If not, after he died, was only one named Hetairos in turn, or could all of them be? If so, it would seem to expand their numbers exponentially. In Dancing with the Lion, I decided to make the formal title apply only to the father/elder brother, but one having it ennobled the whole family. Ergo, Parmenion was the Hetairos, but Philotas, Nikanor, and Hektor are all Hetairoi by extension. I'm never all that clear about it (on purpose), but I had to make some decisions.
In a novel, you can't say, "Well, it might have been this, but it could have been that..." Ha. When reading Dancing, keep that in mind. Sometimes I present as settled a point of detail about Macedonian politics that isn't settled, even in my own mind. I expect most people realize as much, but it never hurts to reiterate the point.
Also, a while back, I did a much longer post on some of the various offices at the court:
TRADITIONAL OFFICES AT THE MACEDONIAN COURT: Hetairoi, Pages, Somatophylakes
