Sak
I've used the same rods in both GL3 and IMX for both gear and fly rods. As Strike Zone mentioned, the IMX is the more sensative of the two.
That being said, I've also broken more IMX rods then any other G. Loomis rods made of different materials. That includes GL2, GL3, GLX, GL4, IM6 and the new streamdance flyrods. I still have IM6 1141's in both spinning and baitcasting that I bought in the early 80's.
The IMX rods are in my opinion more fragile then the GL3's. The IMX rods that I had have now been replaced by GLX's or GL3's via the expeditor warranty program.
Brian