Script to do the optimal compression of a gif or video into a slack emoji. Cropping or limiting the number of frames should be done separately.

gifski-slack.sh

#!/bin/bash

set -euo pipefail

INPUT="$1"
DIMENSION="${2:-128}"

QUALITY=100
TARGET_SIZE=130278 # Not the real value, but I'm too lazy to really test the slack limits

TMPOUT=$(mktemp gifski-slack.XXXXXXX)
TMPYUV=$(mktemp gifski-slack-yuv.XXXXXXX)
MIN=1
MAX=100

trap 'rm -f -- "$TMPOUT" "$TMPYUV"' EXIT

ffmpeg -loglevel warning -i "$INPUT" -vf "format=yuv420p" -f yuv4mpegpipe -y "$TMPYUV"

while true
do
	echo "Trying quality = $QUALITY..."
	gifski --quiet --width "$DIMENSION" --height "$DIMENSION" --quality "$QUALITY" --output "$TMPOUT" "$TMPYUV"
	SIZE=$(stat -c %s "$TMPOUT")
	if [ "$QUALITY" = 100 ]; then
		if [ "$SIZE" -lt "$TARGET_SIZE" ]; then
			echo "Target size reached with maximum quality. Nothing to do"
			break
		fi
		QUALITY=1
		continue
	fi
	if [ "$SIZE" -gt "$TARGET_SIZE" ]; then
		if [ "$QUALITY" = 1 ]; then
			echo "Target size cannot be reached even with minimum quality. Sorry"
			break
		fi
		MAX="$QUALITY"
	fi
	if [ "$SIZE" -lt "$TARGET_SIZE" ]; then
		MIN="$QUALITY"
	fi
	if [ "$SIZE" -eq "$TARGET_SIZE" ] || [ "$SIZE" -le "$TARGET_SIZE" ] && [ $((MAX-MIN)) = 1 ]; then
		break
	fi
	QUALITY=$((MIN + (MAX-MIN) / 2 ))
done

INPUT_FILE="${INPUT##*/}"
mv "$TMPOUT" "${INPUT_FILE%.*}-gifslack.gif"

Update: I'm not sure this works if you are close to the limit: it seems slack always reencodes and might be much worse than gifski in some edge cases. Anyway, it at least helps getting close to the limit.