I love gregtech for my personal use, yet am unsure that it should be included in FtB as-is. It all depends on Slowpoke's and the rest of the team's intent on what FtB is.
Gregtech is a mod that extends Industrialcraft gameplay by adding new tech, and making getting some tech more difficult. It is a mod for hardcore players. It gives a progression excuse to build massive power infrastructures, which is something I enjoy. However if the purpose of FtB is to have the broadest appeal, it may not be good to include it as it is because of the risk of turning off people who don't want to spent loads of time developing such infrastructures, or newcomers who have difficulty figuring out recipes. On the other hand, over time Gregtech recipe differences may become moot, especially if there's a FtB wiki that newcomers can reference easily, instead of getting confused. I really have no clue about the dynamics of learning the mod outside my own experience.
Fine for me
If Gregtech is not included or changed, I can add it myself manually or change configs; I just want to see FtB thrive. The first point to remember in any mod discussion is to think of things not that you want, but what are best for the pack.
Outside of hardcore recipe changes, I do find Gregtech a compelling, fun mod with tons of features I frequently use. The greatest is the Industrial Centrifuge: it's just plain good fun to get different materials from others, and expands options in factory production lines. As just one example of the 60 recipes, you can centrifuge rubber wood and get energy returns almost comparable to turning logs into scaffolds from the methane and plantballs.
The translocators, buffers, and sorters all work wonderfully as non-BC production line options, though they are expensive. That expense is what I would consider a good thing: it allows for the upgrading of your factory to premium high-tech, which is fun to do.
I haven't yet experimented with the Fusionreactor or the rest of the energy tech (beyond the always useful lithium batpacks) but I look forwards to doing so.
So at the very least: leave Gregtech in but change the recipes it changes back, because it adds some legitimately cool stuff anyone would enjoy. Whether or not to change the recipes back depends entirely on the intent of the mod pack.