Is 30 gallon tank okay for Alligator gar ?

not if you want it to grow up. That size tank will only last while its a baby. The local sporting goods store has a huge tank with one in it. I would say its about 1,000 gallons! But for home display I would say a 125gal at least! Most reputable fish supply stores will not even sell you a fish like that untill they are sure you have the propper set up.