“Gig ‘em, Aggies” (with a thumbs-up) is the yell at Texas A&M that was reportedly started by Pinky Downs (‘06) before a football game with the Texas Christian University Horned Frogs.
 
“Gig ‘em” is a term in frog hunting and in fishing, with “gig” meaning “spear.” The term"gig ‘em” has also been used in the military.
 
 
Aggie Traditions
Gig ‘em
At a yell practice before the 1930 TCU game, A&M board of regent Pinky Downs ‘06 shouted, “What are we going to do to those Horned Frogs?” His muse did not fail him as he improvised, borrowing a term from frog hunting. “Gig ‘em, Aggies!” he said as he made a fist with his thumb extended straight up. And with that the first hand sign in the Southwest Conference came into being.
       
Wikipedia
Gig ‘em Aggies is a tradition of Texas A&M University in College Station, Texas, United States.

History
Pinky Downs, class of 1906 and a member of the Board of Regents from 1923 to 1933, is credited with the Gig ‘Em hand sign. At the 1930 Midnight Yell Practice before the football game with Texas Christian University, whose mascot is the Horned Frog, football game, Downs shouted out, “What are we going to do to those Horned Frogs?” Answering his own question, he replied, “Gig ‘Em, Aggies!” while making a fist with his thumb extended. A “gig” is a spear-like tool used for hunting frogs. The gesture became known as the “first” hand sign of The Southwest Conference. [citation needed]

Alternate Story
While Pinky Downs is universally credited with originating the Gig’em hand signal and saying, the story of the origin varies by the teller. The alternate story is that one day Pinky was punishing two freshman cadets for some infraction. While watching the two freshman doing pushups a senior cadet walked by and said “Gig’em Pinky,” in this case a gig meaning a military demerit. Pinky then turned and gave the senior a thumbs up sign.
 
The Mavens’ Word of the Day
The kind of gig you use to catch fish goes back to the Spanish fisga which means ‘harpoon’. The Spanish word may come from the German fischgabel ‘fishhook’, but the connection is not well documented. In any case, fisga first appears in English as fisgig in the 16th century: “Those bonitos…being galled by a fisgig did follow our ship… 500 leagues” (Sparke, in Hakluyt’s Diuers voyages touching the discouerie of America, 1565). Because fisgig is a fishing word, and it sounds a little bit like “fish-gig”, the word is found in the form fishgig as early as the 17th century: “These Fishes are taken with Fishgigs” (Monson, Naval Tracts, 1642).

Fishgig, of course, seems like a kind of gig, one used for fish. It wasn’t long, then, before people shortened the word to plain old gig: “At each end of the Canoe stands and Indian, with a Gig, or pointed spear” (Beveryly, History of the present state of Virginia, 1722). Gig first appears as a verb meaning ‘to fish with a gig’ in the 19th century. And today it can be used to talk about catching fish or frogs (remember this about the frogs, it is important later) with a spear.

So, to get back to your question about the Aggies and their yell, one story has it that the A&M Aggies were playing a football game against the Texas Christian University Horned Frogs when one student (who had apparently stabbed his fair share of frogs with fisgigs) yelled “Gig ‘em Aggies!” Well, that’s one story. It’s a fun story, but in my experience something yelled by a single student at a single football game has about a one-in-a-million chance of catching on. Still, there may be a grain of truth in it. 
 
November 1919, Forest and Stream, “Fish and Fishing,” pg. 18:
“The ol’ gig handle was ‘bout played out,” he began, “an’ so I cut this un. I left it a little longer’n tother, a foot or so don’t hurt none when you’re shinin’ suckers. You can reach further.” The gig, as Matt persisted in calling the spear, was a four-prong affair of rustic build, evidently by the hand of some country blacksmith. The prongs were a trifle more than four inches long with a bar cut near the points. It had been given to him by a man moving from the place and was a cherished object. “I’ve got loads of suckers with her,” he said, after a pause and then proceeded to give Mr. Woodhull a general idea of the process of giggin’ suckers. “You get ‘em sometimes in the day if you’re careful an’ quick, but you have to roust ‘em out from sods’n roots with the gig, nen hit ‘em quick but the best time is nights. They’re o nthe go all night an’ if you shine ‘em right they lay right still. Seems ough you could pick ‘em up with your hand if the water wasn’t too deep. An’ frogs,” he went on, “is perfec’ fools. Shine ‘em right and you can pick ‘em up like a stone.”
   
Newspapers.com
29 November 1928, Austin (TX) Statesman, pg. 28, col. 2 ad:
Welcome Aggies
Beat Those Longhorns
“Gig ‘Em Aggies!”
 
Texas A&M Newspaper Collection
29 November 1933, The Battalion (College Station, TX), pg. 7, col. 2 ad:
GIG ‘EM AGGIES
THE AGGIELAND GROCERY
 
Texas A&M Newspaper Collection
19 September 1934, The Battalion (College Station, TX), “Yell Songs,” pg. 21, col. 2:
Gig ‘em - - - Aggies
 
30 July 1942, Sikeston (MO) Herald, pg. 4:
Now, back to a little more fun, a little song was composed in Squadron 3. The ditty goes as follows:
 
Gig ‘em all, gig ‘em all.
The long and the short and the tall.
Gig all the Seniors, their Officers, too.
Gig the O.D. and the Adjutant, too.
For we’re saying goodbye to them all.
As it’s off to Basic they crawl.
We’re nearly crazy
With all of this hazing.
So cheer up my lads.
Gig ‘em all.