It's impossible to rate a particular pistol's "concealability." There are lots of factors that influence how well you can conceal a firearm, many of which have nothing to do with the firearm itself. Your body shape, typical method of dress, climate, whether you are on your feet or behind a steering wheel or desk most of the day, etc.
That said, the G26 is fairly "concealable" for most men. It is a very small pistol for a 10+1 round capacity 9mm. Lots of LEOs use them as BUGs (back-up guns) in ankle holsters or other out-of-the-way locations.
I don't particularly care for IWB carry with the Glock 26, though. I have one, and for the first year after I got my HCP (Tennessee handgun carry permit) it was what I carried. I always carried IWB. After about a year, I switched to a 1911.
The reason I switched is that the Glock, while small, was small in the wrong dimensions. It is compact in height and length, but not in width. Both the slide and the grips are fairly thick, and it has a very boxy contour.
The 1911 I now carry is actually thinner in the slide than the Glock, despite being .45 caliber. The slide is the part that is actually inside your waistband when carrying IWB, so slide thickness is a critical dimension in terms of comfort and concealability. My 1911 has a commander length (4.25") slide, so it is quite a bit longer than the G26. All that length is hidden, however, when you carry IWB, so it is easily concealed. The longer slide gives longer sight radius, better reliability and higher velocity, too.
My 1911 is also slightly thinner through the frame than the Glock, thanks to it being single stack .45 vs. the G26's double stack 9 mm. The grip frame profile is also much more rounded (as is the top of the slide) than the Glock, which makes it less likely to jab my side when I move and much less likely to "print" through clothing.
Don't get me wrong -- I like the G26. I just found that I can more comfortably carry a 1911, at least IWB, and I prefer 7+1 rounds of .45 to 10+1 of 9mm. The 1911 mags are also MUCH thinner than G26 mags, so it is easier to slip an extra mag in a pocket. I kept my G26, though. It's a good, reliable, light, small gun.
YMMV.