Validity of GUI design patterns under linux
Given how you explain your application in a comment, while fully endorsing Qt, I would also recommend you consider the many advantages that could come your way from making your application a web app.
Since you say it's a client-server app, it needs (at least) local network connectivity at least, so the first objection typically raised against web apps is nullified.
The first huge advantage is that you wouldn't be choosing one client platform vs another -- just support modern cross-platform browsers like Firefox or Google's Chrome, and your customers will be able to pick whatever client platform(s) they prefer (if you also carefully check your app on Safari, which has much rendering logic in common with Chrome via the Webkit framework, your web app will then be usable on iPad too).
The second big win is that your app won't require any "installation" on the client(s) -- it will always be ready. 
Modern Javascript frameworks (such as jQuery, Dojo, Closure, ...) allow heavy interactivity if you need it, support GUI building with UI widgets &c, and incidentally take care of most cross-browser differences on your behalf.  On the server side, with either Ruby or Python (or other languages yet), you can even find frameworks that smoothly integrate with the client-side Javascript resources.
Oh, and, the computational resources needed to run the application (such as RAM, CPU power, disk space, ...) are cheaper "in bulk" on a server, or small group of servers, and thus shared among those clients which are active at a given time, than spread out over many clients (including many who won't happen to be active at any givem time;-).
Really, in my opinion, there is little left today to recommend local GUI apps when you're developing anew anyway, save possibly the need to run when cut off from all connectivity (and, even there, with HTML5 and the like, browsers are making great strides towards empowering such apps).
I am investigating a switch from windows to linux and i struggle to find samples of design patterns applied in this world.
The application is a classic client server with detailed forms for each business entity.
It will be taking user input, do some validation checks, a few calculations, and save them to database.
It will also have lists summarizing entities, and searches among these entities.
A click to one item of these lists will open the detailled forms.
If i use python or ruby, what should i use for GUI ?
And what pattern : is MVC, MVVM or MVP any good ?
Or is there anything better ?  
Note : I have never done web developpement, and i would like to avoid to have to learn both linux and web developpement at the same time.