Whichever programming language can be used for any activity. Of course some are better for one type of application than others.
There are some languages that have chosen to be niche themselves and are very good at it. An example is R which is suitable for statistics.
But the vast majority of languages choose to be in general use to reach a larger audience. Of course they are not usually better than others at anything, they are just reasonably good at everything, it is the case of Ruby.
What often determines a slightly greater use of the language is not the language itself, but the platforms where it starts to be used or the existing libraries for it.
Clear examples of the first case are Javascript, which was used in all browsers and even standardized, and Lua, which was chosen by many games as a customization language.
In the second case Ruby is the best example where the library determined the success and niche of the language. There are people who think the language is called Ruby on Rails.
But the reality is that Ruby can be used for anything that does not require extreme performance. Ruby is not known to be a fast language. And as much as there are tools to make it faster, there will always be limitations, you can never compete with static typing languages, just to be in the most obvious example.
I don’t know how Ruby support is on mobile devices. I believe it is weak or non-existent. That may change, but I doubt there’s still room for Ruby to be strong in this. Note that even if full support will be by library responsibility and not language.
Want to make GUI/Desktop applications? Ok. Ruby supports through existing libraries such as: Shoes, Fxruby, Qtruby, etc..
Want more? Search on Ruby Toolbox and Ruby Gems. There are literally thousands of libraries, most of them are not for the web.
I only use Ruby to create desktop programs, with graphical interface and everything. Type if you download and install Ruby, you can already start programming for desktop. It runs at command prompt. Already to create websites, I think you need to do something more.
– user110265