I just want to know if i can write GUI apps for Ubuntu using C--again, not C++. It's just that I hate having to learn a new language entirely (I know that they are similar in some ways but still). Just want to know thanks in advance.
You can develop apps for Ubuntu with just about any language. If there are particular libraries you need your app to use, then as mdpc says that's the only thing that would likely impose any restrictions. Some libraries can only be used from certain languages.
But Ubuntu apps can be developed in C, and a lot of Ubuntu softwatre is written in C, including many programs and libraries installed by default in almost every Ubuntu system.
Programs that are installed by default in Ubuntu or otherwise very popular, and are written in C, include:
So yes, you can definitely develop for Ubuntu in C. Many people do.
With that said, the Ubuntu SDK uses Qt. Qt has bindings for many languages but not C. So you probably cannot create programs with the Ubuntu SDK, by coding solely in C. (You could still often code most of your software in C.)
That may make it difficult to write mobile apps (for Ubuntu Touch) in just C. But there's no need to use the Ubuntu SDK for desktop apps. Many desktop apps (all but 4 apps listed above are desktop apps in the usual sense) that are packaged for and run well in Ubuntu don't use the Ubuntu SDK.
Furthermore, using the Ubuntu SDK may not be the best way to create portable apps that run well on other desktop operating systems besides Ubuntu. The Ubuntu SDK is good for many purposes, but even if it did support writing apps in C, you'd still likely not use it under a variety of circumstances.
One popular way to write GUI applications in C is to use Gtk+. There are other alternatives for GUI toolkits with C bindings, including WxWidgets (via WxC), IUP, and possibly Clutter (GObject is accessible from C but sometimes writing for its object model--or any object model--is difficult in C).