Variables in the workspace are initialized once during model update before the simulation runs. Therefore, a change in value of the variable in the workspace will not reflect a change in value of the block parameter during simulation.
You can however change the value of the parameter directly by assigning the value of the workspace variable to the block with the following command:
set_param(gcb , 'rep_seq_y' , ['[' num2str(a) ']'])
Where 'a' is some row vector.
Alternatively, after you change the workspace variable, you can force a model update to ensure any changes in workspace variables are reflected in the model during simulation:
set_param(bdroot , 'SimulationCommand', 'update')