In most cases, you can simply just return 204 status code (for empty response) to shave away the bytes used to serve 1X1 pixel.

The pixel is simply there to make the request.

You can do a png instead of a gif. It would be ~68 bytes instead of ~35 bytes. A webp image would actually be 1 byte smaller than the gif. So perhaps Google will move to that if Safari ever caves and supports it.