Create a Square Wave Generator using Level-2 S-Function and MATLAB Function block in Simulink. 

I understand that you want to know the block for loss (opposite to gain block) in Simulink. I don't think there is any specific loss block. However, to implement loss you can use gain block with value in fraction (less than one). This value of between 0 & 1 will make the gain block act as loss block as it will decrement the input signal resulting in loss.

Initial Conditions 𝑦'( 0 )= −1 y( 0 )= 1

You can check my video: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-HR9KqHOiszc.html In this video I explained the steps to design the differential equation in Simulink using basic blocks. I hope it helps. Good Luck :-)