Ni Manoj,
Well, there is a HUGE difference between Florida Gars and Alligator Gars.
Alligator Gars are the among the fastest growing monsterfish out there and also one of the most aggressive.
My Gator Gars went from 4 inches to 18 inches in about 4 months. They also ate two 8" Bala Sharks in their tank.
This is the thread documenting their size when I got them
More pics in a WTS post from back in Nov 2010. They grew another 4-5 inches in that one month (Nov 2010)
I would strongly recommend looking for a buyer for your Alligator Gar ASAP and selling them off when they are small.
Your silver aro will not survive in the same tank as the Alligator Gar if they are of the same size right now. (Gator Gar will outgrow the Aro and out compete the Aro for food.)
Good luck,