I would like to have a OpenModelica model like the following:
On the left side there is a "Real Input Connector" called u and on the right side there is a "Real Output Connector" called y. The Porpose of the model is just to take a value (u) multiply it with 2 and give back the output (y).
Now my questions: How do I set a value to the input variabe u?
Is the input connector the right block to do this?
This is valid if you want this gain to be a standalone submodule that you'll connect to other models. In that case you need to connect the output of a source to your input block. The standard library provides a variety in Modelica.Blocks.Sources, or you can make your own.
As a trivial example, with your example saved as My_Gain.mo, to connect to sinusoidal source:
model SampleModel
My_Gain gain;
Modelica.Blocks.Sources.Sine source;
equation
connect(source.y, gain.u);
end SampleModel;
Without knowing more about what you're trying to do, it's hard to say if this is the right way to go about it, but it will work.