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Different crash now. Gendustry (and possibly its dependency, bdlib) needs to be updated to handle Forestry 4.x - presumably you had an older version of it to work with Forestry 3.6. Binnie's Mods and Magic Bees would have similar problems, but it looks like you're not using those mods.
Okay, looking at GitHub code for Forestry, it seems the static "instance" field of the AlleleHelper class that GT5u is looking for only exists in Forestry 4.1 and most newer versions (thus apparently I unintentionally misled you by saying you needed to update to "Forestry 4.x" ) IDK how other mods were expected to be able to use that AlleleHelper class without a static instance field in older versions, though.
And to get Forestry 4.1 I would need to update forge too
I haven't kept track of exactly what version of Forge different versions of mods need, and I hadn't noticed until now that you were using build 1492 of Forge, so now I'm curious why. If you're going to customize an old modpack (Tekkit Legends was last updated 2 years ago, now that I look for it) by adding a much newer mod like GT5u (the modpack doesn't include any version of GregTech by default), it seems to me you might as well update Forge and all the other mods to their latest MC 1.7.10 versions while you're at it.