It's better to using the regular version 5.09 of you are not using my whole pack. Maybe you run in errors because some mods and materials are missing.I modified Gt 5.08 a bit but only for the Modpack Gt new Horizons. New Materials are added like from Ender Io Draconic Evolution or Stevens Carts. A ebf tier 3 is added and more larges cells. In the core mod most of Gt machines in the Luv Zpm and uv category are added. We have new batteries for this tiers and new crafting items like motors and conveyor belts. A new low tier tin pipe for items and many more.
The new features like the new machines from 5.09 are not included but I am planing this soon make a custom Gt 5.09 version.
Nice to see, that you plan to update your pack to the 5.09 branch. Is there any information about when we can maaaybe expect this to happen? I do plan to setup a server with your pack and I would really like to use the new version in this, so I will patiently wait