Setting up IC2 and GT API

1. Download these (don't extract)
http://files.minecraftforge.net/maven/com/gregoriust/gregtech/gregtech_1.7.10/6.00.50/gregtech_1.7.10-6.00.50-dev.jar for GT API (version 6.00.50 for 1.7.10)
http://files.minecraftforge.net/maven/com/gregoriust/gregtech/gregtech_1.7.10/6.00.50/gregtech_1.7.10-6.00.50-sources.jar for GT sources for same version
http://ic2api.player.to:8080/job/IC2_experimental/lastSuccessfulBuild/artifact/build/libs/industrialcraft-2-2.2.767-experimental-dev.jar for IC2 API (version 2-2.2.767-experimental)
2. Put the APIs in your mods folder, but not the sources (minecraft/eclipse/mods), put the sources somewhere where you can find them.
3. In eclipse, right click the Minecraft project in the package explorer, click properties
4. Go to Java Build Path
5. Click on "Add External Jars"
6. Go to the mods folder and select both the IC2 and GT APIs then click open
7. Expand the GT API, double click on Source attachment
8. Select the External location radio button
9. Click external file
10. Select your GT source files
11. Click OK, and then click OK on the properties screen
12. After rebuild, you should be able to refer to GT and IC2 API methods.

If you need references to the APIs, here's the IC2 manual: http://ic2api.player.to/ic2api/html/index.html
With GregTech, I am unsure if there's a manual, but because you linked the source file, you should be able to look at the source code for GT from within Eclipse.