The Black Diamond Gridlock is a popular belay carabiner, because it has a clever mechanism that (in theory) prevents your carabiner from being cross loaded. You would think the narrow end with the clip around it should always be put in your belay loop, right?

Apparently, not always. At least, that what Black Diamond says.

Lucky for you, the Alpinesavvy web gnomes actually read instruction manuals from time to time, even for carabiners.

The following is a screen grab from a PDF from the Black Diamond website, which you can see here. (You'll either need a magnifying glass or hit “control +” on your keyboard about seven times to make it big enough to read.)

Check out the first two diagrams. With the tube style belay device and a munter hitch, Black Diamond wants you to clip the carabiner in the logical way, with the small side toward your belay loop. 

But look at the third diagram, with a Grigri. According to this, you're supposed to turn the carabiner so the small side is AWAY from your belay loop.

I don’t work for Black Diamond, so I can’t comment on the exact reasons for this. But, judging from the last diagram on the right, it appears that cross loading is a concern. Keep in mind the whole point of using a carabiner like this is to avoid across loading . . .

(And yeah, it’s a bit odd that BD is telling you how to use a belay device made by Petzl, but that’s a different conversation.)

Alpinesavvy offers ideas and information, not advice. If you use a Gridlock carabiner with a Grigri, consider yourself informed as to what Black Diamond wants you to do. Following manufacturer recommendations is usually good practice. But, you're free to use your belay device any way you like.