You can use a Constant-Block (name it myRandInput) and change the value of this block over the OpenFcn (open function callback) of an empty Subsystem. (Right-Click on the Subsystem -> Properties -> Callbacks)
Or you can use an annotation ClickFcn. (Double-Click into the model -> Name the annotation: e.g. ClickHere -> Click outside the annotation -> Right Click on the annotation -> Properties -> ClickFcn)
In this functions you can create a random number and write it to the Constant value.
myValue=rand;
modelName = gcs;
set_param([modelName '/myRandInput'],'Value', num2str(myValue));
Note: The Constant-Block is named: myRandInput
