The best way to have a good system in GUI... is JASS or vJASS
...
See, JASS is not deep JAVA, C++, BBcode, HTML thing... Is the same than GUI, but more flexible, and without all the -eye-candy- stuff. Besides, you don't have to 'Do JASS' the only thing you need to do is follow instructions.
GUI: Event Response - Picked Unit
JASS: GetEnumUnit()
...Was that so hard?.
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Once upon a time, I knew nothing about World Editor... two months later, I now GUI, a really small part of JASS and vJASS... not as to create something, but enough to modify here and there a bit (as most JASS based system allows). I had a lot of problems to, but I feel like in heaven now.
I started just like you: "I want everything in GUI"... Now, I still use GUI, but I use a lot of customs scripts (JASS) in the GUI Triggers... And whenever I want to know how something in GUI is done in JASS, I just create the trigger in GUI, create a copy of it, and convert the copy to custom text (Select Trigger -> Edit -> Convert to Custom Text) then I check the GUI trigger, and see how the same action is written in JASS.
Don't be afraid, it won't eat you, nor bug your map. Create a backup, and follow instructions.
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Download
JassNewGenPackv5d and last
JassHelper version
Install instructions: Insert the JNPG (JassNewGenPack) folder into your Warcraft Directory (The folder, not the files). Extract JassHelper 0.A.2.B somewhere, and replace the jasshelper.exe (In the JNPG\jasshelper folder) with the fresh extracted jasshelper.exe. (Forget about the other files)
Use NewGenWE instead of common WE.
If you find any problem, ask in the forum. There's a bunch of lovely people there that will be pleased to help you