Hey! So here is what I can see. First the obvious, the gentleman in this picture is a Sergeant First class.

He is a Field Artilleryman or a combat engineer. This is indicated by the red stripe around his service cap. The Field Artillery color is scarlet and the engineers are red and the difference is very subtle and I cannot really tell which.

The cord around his left shoulder is called the french Fourragere. It is a unit award given to units by the french government, usually for performance in WW2. Not that the gentleman here fought in WW2, only that his unit did and performed well.

Now for a bit of conjecture, I am a Field Artilleryman. The only FA unit I know of that has recieved a french fourragere is 1-76 Field Artillery. This unit is currently based in Fort Stewart Georgia and recently reflagged to 1-9 FA. This unit has a history and lineage section and they may or may not have more information on your granpop. A longshot, but might be worth it.

As for his ribbons, he received an Army Commendation medal, a Good conduct medal, Legion of merit, I think I see a Republic of Vienam Armed forces honor ribbon, an Army Achievement medal, that is definitely a Republic of Vietnam Campaign medal, and a couple others I cannot really tell.

The three silver medals are various proficiency badges for different weapon, IE qualifying marksman on a rifle.

He is wearing a bow tie meaning he is attending a military ball, or if he is a field artilleryman, he might be at a Saint Barbara's Ball.

here is what I find odd about this photo. This is your great grandpa, and it apears "old" but he is wearing ASUs. The ASUs were not introduced until 2008. Yet his ribbons are from Vietnam. During vietnam, and all the way up until 2008, soldiers wore the Army Class A Greens. "The Green Suit". So I found that odd. Unless he was still in the military in 2008? Or maybe he went buy himself a modern uniform to throw his ribbons on for this picture? Not sure.

Hope some of that helped!