If you don't want to use something more composed like this, the main trick gifski is doing is using a two-pass approach to generate a palette for the gif. You can do this pretty easily with just ffmpeg / ffprobe. Here's my personal script I use for generation:

    #!/bin/sh

    SCALE=500
    FPS=15
    SCALEPROVIDED=false
    
    for arg in "$@"
    do
        case $arg in
            -s=*|--size=*)
            SCALE="${arg#*=}"
            SCALEPROVIDED=true
            shift 
            ;;
            -f=*|--fps=*)
            FPS="${arg#*=}"
            shift 
            ;;
        esac
    done
    
    palette="/tmp/palette.png"

    # Look at the width/height of the video
    width=`ffprobe -v error -select_streams v:0 -show_entries stream=width -of csv=s=x:p=0 $1`
    height=`ffprobe -v error -select_streams v:0 -show_entries stream=height -of csv=s=x:p=0 $1`
    
    # Get the max
    max=$(( width > height ? width : height))
    
    # If we havent provided a scale, and the max size is lower than the default, set it to the lower value
    if [ "$SCALEPROVIDED" == true ]
    then
        minScale=$SCALE
    else
        minScale=$(( SCALE < max ? SCALE : max))
    fi
    
    # Set our flags
    filters="fps=$FPS,scale=$minScale:-1:flags=lanczos"
    
    # If there's no name provided, output the same name as the input with gif as the extension
    output=$2
    if [ -z "$output" ]
    then
        output=${1%.*}.gif
    fi
    
    # Output our gif
    ffmpeg -v error -i $1 -vf "$filters,palettegen=max_colors=256" -y $palette
    ffmpeg -v error -i $1 -i $palette -lavfi "$filters [x]; [x][1:v] paletteuse" -y $output
    
    echo created $output at ${minScale}px

Example usage is something like:

    gif input.mov

But you can also optionally specify framerate, size, and output name:

    gif -s=800 -f=22 input.mov my-800px-22fps-gif.gif