Hello,
To my understanding you are trying to multiply flow rate with some contant or predefined quantities. You are tying to achieve this my using product or gain block. To use a gain/product block in series with a hydraulic Simscape model, you can use the 'PS-Simulink Converter' or 'Simulink-PS converter' converters for this purpose. You can visit the documentation links for them for more information.
Hope it helps
