Dude I am pretty sure it won't work because I am going through a similar situation. I had 2 red devil cichla because someone gave them to me. When I started a thread here asking how that would work I found out quickly it was no bueno, which is spanish for not good.
Check out the thread so you can see since this relates to your situation:
http://www.monsterfishkeepers.com/forums/showthread.php?p=3886314&posted=1#post3886314
Also, after finding this video of the fish you have online, your Gar is in danger being so small still:
http://www.youtube.com/watch?v=b_4CfBi6TWU
The Gator Gar is a WAY better fish than that jag thing, so just ditch the jag and enjoy the powerful beauty that is... The Alligator Gar.
Actually, here is not only a great idea but what you should tonight/tomorrow.
You're going to need a larger tank for your Gar very soon anyways. So I would say just go get that larger tank and house the Gar in there. Then you can keep that other jag thing in the 75
THEN... when the Gator Gar get to be about 3 feet or so then you can reintroduce them