See script and documentation here: https://github.com/troyane/StackOverflow-pro/tree/master/mp42gif Script to convert MP4 file to GIF (via ffmpeg). It creates intermediate custom color palette out of input video file and use it for resulting GIF for better picture quality. For more info see https://superuser.com/a/556031

mp42gif.sh

#!/bin/bash

#
# Script to convert MP4 video to GIF with generation of custom color palette.
#
#=== Do not touch code below
# Inner variables
input_file=""
input_fps="20"
input_height="512"
output_file="output.gif"

# Great and simple idea got here: https://stackoverflow.com/a/53463162/867349
function cecho() {
    RED="\033[0;31m"
    GREEN='\033[0;32m'
    YELLOW='\033[1;33m'
    NC='\033[0m' # No Color
    printf "${!1}${2} ${NC}\n"
}

function wecho() { # warning, yellow
    cecho "YELLOW" "[WARN] ${1}"
}

function eecho() { # error, red
    cecho "RED" "[ERROR] ${1}"
}

function iecho() { # info, green
    cecho "GREEN" "[INFO] ${1}"
}

if ! [ -x "$(command -v ffmpeg)" ]; then
  eecho 'Error: ffmpeg is not installed.' >&2
  wecho 'Install it by next command "sudo apt install ffmpeg" and run script again.'
  wcho 'Exiting.'
  exit 1
fi

function usage() {
    cat <<HELP_USAGE
    -i|--input   Input MP4 file. 
    -f|--fps     FPS frame per second rate as a number. Default FPS=20.
    -h|--height  Height value as a number. Script will scale input picture height to given number. 
    			 Default Height=512. Width will be calculated proportionally.
    -o|--output  Output GIF file. Default Name=output.gif.
    --help       Show this usage info.
HELP_USAGE
    exit 1
}

function parse_args() {
    if [ "$#" -eq 1 ]
    then
    	eecho "You need to specify arguments."
    	usage
    	exit 1
    fi
    while [[ "$#" -gt 0 ]]; 
    do 
        case $1 in
        	-i|--input)  input_file="$2";   shift;;
        	-f|--fps)    input_fps="$2";    shift;;
        	-h|--height) input_height="$2"; shift;;
			-o|--output) output_file="$2";  shift;;
			--help)      usage ; exit 0;;
            *) eecho "Unknown parameter passed: $1" ; usage;;
        esac
        shift
    done
}

function convert() {
	palette_file="palette.png"
	if [ -z "$input_file" ]
	then
	      eecho "No input file given."
	      usage
	      exit 1
	fi

	iecho "Create palette $palette_file out of $input_file"
	ffmpeg -i "$input_file" \
		   -vf fps=$input_fps,scale=$input_height:-1:flags=lanczos,palettegen\
		   $palette_file
    if [ $? -eq 0 ]; then
       iecho "Palette created."
    else
       eecho "Can't continue. Exiting."
       exit 1
    fi

    iecho "Create GIF file ($output_file) based on input file ($input_file) and palette ($palette_file)."
	ffmpeg -i "$input_file" \
		   -i $palette_file \
		   -filter_complex "fps=$input_fps,scale=$input_height:-1:flags=lanczos[x];[x][1:v]paletteuse" \
		   $output_file
    if [ $? -eq 0 ]; then
	    iecho "Output file created:"
	    ls -lah "$output_file"
    else
       eecho "Can't continue. Exiting."
    fi

	rm $palette_file
}

# Establish run order
function main() {
    iecho "Starting..."
    convert
    iecho "Finished."
}

parse_args "$@"
main

Thanks! Works perfectly!
Tip: For windows users, just open this file with notepad, edit the parameters and run it with "Git for Windows".

Thanks! Improves the gif quality highly.