That's one big fish, even making allowances that the man in the picture may be further away than he appears, giving a wrong idea of the scale. Here is a comment on Alligator Gar by Joe Zentner ("Fishing for Living Fossils", Florida Wildlife Magazine, Mar-Apr, 2009).

"Reports of 12 to 20 foot long alligator gar specimens, are probably exaggerated, but the fish's actual size is amazing enough. One of the biggest alligator gars on record measured 8 feet, 5 inches. It weighed 356 pounds and was taken from Arkansas Horseshoe Lake in 1931. A specimen taken from a bayou in Vermilion Parish, La. was, it is claimed, 9 feet, 11.5 inches long and and weighed 387 lbs.

I saw one from a canoe in Lake Panosoffkee, Florida, that I first mistook for an alligator. It passed by the 14-foot canoe like a slow-moving torpedo, and seemed even bigger than my boat, but it was several feet under water and its size was no doubt magnified by the water, so it was hard to estimate accurately. I never did have both ends of it in sight simultaneously. It did have a rather large, and violently struggling, softshell turtle in it's jaws, though.

Here are some other records from a sportsman's web site.
http://www.landbigfish.com/staterecords/fishrecords.cfm?ID=67

Alabama 151 lbs 5 oz Tensaw River Michael Houseknecht 8/13/2004
Arkansas 240 lbs 0 oz 8'2"" White River John Stortz 7/28/2004
Florida 123 lbs 0 oz Choctawhatchee River Zachary Phillips 7/8/1995
Mississippi 215 lbs Mississippi River near Natchez Earl Stafford 5/3/2003
Oklahoma 184 lbs 3 oz Red River Sean Chatham 2/28/2006
Texas 279 lbs 0 oz Rio Grande River Bill Valverde 1/10/1951