Set requires_grad to False for which parameter you want to freeze.
Like
# In SimpleGaussian.__init__()
self.conv1.weight.requires_grad=False.

Also, I recommend you to inherit Conv2d class for GaussianBlur class because what it does is just a convolution.
Like
class SimpleGaussian(nn.Conv2d):

In this case, you don’t have to redefine forward function and it will make your code much simpler.