I have an application that I want to be MIT and I have a roo gem in my Gemfile (roo is licensed under MIT) however it depends on spreadsheet that has GNU license. Am I allowed to use roo and have MIT licence?
The MIT license is compatible with the GNU GPL. You can use MIT libraries in your projects, so long as you continue to abide by the GPL's terms.
However, the MIT terms only apply to the MIT library in isolation. If, for example, you want to close your source, you cannot do it if you use any GPL'd code in your project. The copyleft provision of the GPL takes precedence over the "use it however you like" terms of MIT.
So even though you're using MIT code that is compatible with the GPL, you still must abide by the responsibilities of the GPL.