some info, easist is to get "Grab" back from a backup, time machine or from any pre-mojave running machine via "copy > paste" "drag and drop etc. etc.
https://www.makeuseof.com/tag/best-gif-apps-mac/https://itunes.apple.com/us/app/gifgrabber/id668208984?mt=12https://itunes.apple.com/us/app/gif-brewery-by-gfycat/id1081413713?mt=12Restore Grab from a previous macOS Time Machine backup, if available
Open Time Machine or open your backup drive’s backups.backupdb folder
If using Time Machine, go to a past snapshot from a previous macOS version like High Sierra or Sierra
If opening your backup folder, select a backup date before you updated to macOS Mojave
Locate Grab in the Utilities folder inside the Applications folder of your backup
If using Time Machine, command-click on Grab and select Restore “Grab” to… and choose your Applications > Utilities folder as the folder for the restored item
Enter your admin password, if asked
If searching your backup drive without Time Machine, locate Grab and paste it in your Mac > Applications > Utilities folder
Once complete, verify that Grab is on your Mac running macOS Mojave in the Applications > Utilities folder, open it, and conduct a test screenshot using Grab
You can also copy it from another person’s Mac running macOS Sierra or High Sierra or from their Time Machine or another backup macOS Sierra or High Sierra.
if someone wants/needs that we find ways how you can get one from me, i have 1 of 4 macs running on high sierra and can post a dropbox link here:
Via the link below you should be able to download "Grab" from my dropbox
https://www.dropbox.com/sh/chdlu6q3o0rizz8/AACExYdgybIKSJVSQyRPqYbWa?dl=0