For reference you can see this in action in Empire of the Sun 5.0 v7. To summarize:
I have a command for the Game Turn marker to shift to the next space on the turn track (Turn 2, Turn 3, etc.) with a Global Key Command (as part of the end of the turn cleanup) applying only to this marker (Matching Properties: BasicName==“Game Turn”). This trigger’s the Game Turn marker’s Send to Location command to calculate the next game turn position.
This works absolutely fine with the GKC as long as the turn track spaces are empty, but if I have other units or markers on the turn track (say reinforcements or whatever) that the Game Turn marker can stack with the Game Turn marker will “Hop” until it lands in the first empty game turn space. Say for example it starts in Turn 1 and I put in a unit in Turn 2 and Turn 3, my expectation is that the Game Turn marker will move to the Turn 2 space and stack with the unit in the same Turn 2 space, instead it goes all the way to Turn 4!
Using the Report Action, it seems to trigger the Send to Location for each pre-existing stack it enters, which is very weird to me and unexpected. Am I doing something wrong with the GKC?
GKC’s are super-tricksy this way, and their interactions with movement – pieces that move to another location can end up “getting back in line and receive the command again”! (Likewise I have had a GKC that was supposed to go to both Piece A and Piece B end up not getting to Piece B, because Piece A moved Piece B during the GKC).
Couple ways you could fix this–
METHOD 1:
(1) Your end-of-turn cleanup (wherever that is, that’s one thing I didn’t go find) updates the Turn Global Property (increments it) BEFORE sending the GKC.
(2) Turn marker then receives GKC and does not need to update Global Property (so does not risk a multi-hop if it receives GKC twice), it just goes to what turn Global Property tells it to.
(so now, see Turn marker is “hardened” against receiving the GKC twice)
METHOD 2
(1) Have a secret invisible piece on the map somewhere.
(2) Send THAT piece the original GKC from your end of turn cleanup.
(3) THAT piece then updates the Global Property for Turn, and then sends a GKC to the Turn Marker to find the right turn
(4) Turn marker receives that GKC and puts itself on the right turn spot.
(again, Turn marker is hardened against two GKC’s)
METHOD 3
(1) You could add one of those fancy “Turn Counter” widgets to your toolbar, and THAT can keep track of the turn number (and it will update the global property for you)
(2) And then it will send a GKC to the Turn Marker when you want it to.
… You can see this method at work in For The People or Paths of Glory, latest editions. The fanciest version is the one in For The People, which also allows the marker to be manually dragged and still keeps the Turn Tracker up to date. The PoG version is a little simpler, so just depends on whether you’re looking for simple or elaborate 
Someone who has as many global prototypes as you have in there, I’m guessing maybe elaborate haha!
FINALLY… I wanted to mention that Empire of the Sun is an AWESOME module. Let’s just say that I … picked up a couple tricks there!
Brian
I forgot to reply back in here! Thanks for the insight and ideas Brian.
An hour after posting I ended up just keeping track of when the Turn Marker moved the first time and stopped it there. I do have a number of invisible “command” pieces that I could probably assign the game turn update and then trigger the Game Turn move. I’ll look into that.
I probably like prototypes way too much. I haven’t found a proper balance between too many and not enough. I also think I have gone overboard with unit layer sandwiching but hey it allowed to me do a lot of neat stuff.
And thanks for the Kind words on Empire of the Sun. I’ve also picked up ideas from lots of modules I have looked at. I haven’t felt the need to use the Turn counter widget but I may look at that.