The internal rule decides the data type of the Gain parameter based on the input signal's word-length and sign, and attempts to maximize the precision. In this particular case, the input data type is Integer and therefore a precision-loss warning is generated.
To work around this, you will need to manually specify the data type of the Gain parameter. To do this, open the Gain block's mask dialog and click on the Parameter Attributes pane. In the drop-down menu that appears, select the appropriate data type for the Gain parameter.