I had no heard of ECT, but I'm not impressed. I've just benchmarked it against two others PNG optimizers, and here are the file sizes for default and max levels:

    1985457 oxipng-o6.png
    2030036 oxipng-o2.png
    2125459 ect-o9.png
    2144598 ect-o3.png
    2169351 optipng-o7.png
    2215086 optipng-o2.png
    2218326 original.png

    oxipng 9.1.5
    OptiPNG version 7.9.1
    Efficient Compression Tool Version 0.9.5

BTW, I could not compile ECT on my Linux system, because its CMake config was too old. I used the Windows release through Wine, but it shouldn't change the results above.

I tried to apply ECT to a few .gz files, but it complained it was not compatible, and I did not dig further.

[edited for a typo s/I/it/]