As the error suggests, the RunTimeHandle.OutputPort(1).Data (or the Data property of any port) is read-only, so you cannot set it from the GUI. The only way to set port data is if you are writing an S-function block and computing the output signal.
I think we may be able to suggest a different solution depending on what your application is. If you simply want to override a signal with a constant value, then you may want to consider adding 2 Constant blocks and a Switch block to your model.
---------
Signal to override ------|> |
| Switch |---------> to other blocks
Constant1 -------|-| >=1 |
| |
Constant2 ---------|> |
---------
Initially, your Switch will have the "Signal to override" selected (Constant1=1). In your GUI callback, set the value of the Constant block using:
set_param('modelname/Constant2', 'Value', '5');
set_param('modelname/Constant1', 'Value', '0');
set_param('modelname', 'SimulationCommand', 'update')
The second line makes it so the value of Constant2 is selected as the output of the Switch. The third line forces Simulink to take note of the new parameter values.