All vanilla blocks are showing the mod name as "GuiAPI"

Btw: Which Minecraft version and which GuiAPI version are you using?
Also: If you are using a version that is affected by the Class Loader bug, have you installed Class Loader Fixer?

Edit: And which Launcher are you using (e.g. MultiMC or "Vanilla" or BetaCraft etc.)?

I am using.. Minecraft version 1.4.7. I am using v0.15.1 of GuiAPI

I was not using Class Loader Fixer. There does not appear to be a version for 1.4.7, soI tried installing version 1.4.5-1 it into my jar but now I get the following error

java.lang.SecurityException: SHA-256 digest error for cpw/mods/fml/relauncher/RelaunchClassLoader.class

I use SKCraftLauncher but I also tested it in MultiMC

Addendum: I was able to replicate the issue with 1.4.5, but not with 1.4.7.
Will be fixed in 1.0.1 whoch should be released at some point this week!

0.15.1 of GuiAPI is appearently for 1.4.5. Are you sure that is the version you installed?

Yes I'm sure that I am using GuiAPI 0.15.1 on 1.4.7

1.1.0 has been released a few minutes ago and includes a fix for this issue :)