The original G15 keyboard is nice.
18 G-Keys.

The newer G15, has six G-Keys I believe.

Both have M1/M2/M3, which load/unload a set of G-Keys.
So G1 does one thing with M1 enabled and something different with M2 enabled.

I don't actually use the default values for my G-Keys.
And instead use them as hot keys for an assortment of mapped keys.

They are an extra tool, more keys to assign.
In the category of Nice but not Necessary.
You could set up a series of Clickbars to duplicate the G-Keys.
They would be clicked on the screen, via the mouse.
But would essentially be unlimited extra hot keys, well subject to screen space.