The triple conversion is a natual on these bikes, since the bb spindle is already a triple length to accomodate the two-rings-plus-chaingard that was fitted to the GS. The chain gard was actually a toothless sprocket that was fitted to the normal big ring position.

Looks like the owner put the small ring to the granny position, moved the big ring to the normal outer position, and added a middle ring of perhaps 46t. The extra-long bolts and the 3 spacers needed to secure all 3 rings were already on there when it was originally built and sold as a double.

The frame sizing is a concern though, since these GS bikes have a layed-back seat tube angle of only72 degrees.
This will make the top tube feel even shorter than it is whenever the rider gets up off of the saddle, so one's knees will be too close to the bars.

I found this one "Grand Sports" about 9 years ago for $80, and did a pretty complete rebuild on it, a lot of work in other words. It does have quite steady handling.
I even built an "ultra-6" speed freewheel for it using a Dura-Ace 7s freewheel (with the 12t cog removed). I also removed the "toothless sprocket" chainguard from the crank, and so also replaced the bb spindle and chainring bolts with parts from a double.

Some of the GS's used claw-mount derailers, but mine has Zeus dropouts with hanger that seems to bend quite easily.
I had to replace the original 26.8mm seatpost with another 26.8mm post that measured just a hair bigger, since the original post wouldn't hold it's position over time.
This bike measures about 57cm, but feels more like a 54 or 55cm frame to me, and the BB is low, so the head tube is shorter than expected.