I would like add a meme or gif in my app. I find (https://giphy.com/) for gif or (https://api.imgflip.com/)
Ionic 4 is just a web-app when it comes to rendering images or videos or gifs.
All you need is to have valid gif file url and use img html tag to render it:
<ion-header>
<ion-toolbar color="primary">
<ion-title>
Ionic 4 template
</ion-title>
</ion-toolbar>
</ion-header>
<ion-content padding>
<img src="https://media.giphy.com/media/WOwRUGNtWhp19Jaep0/giphy.gif">
</ion-content>
You can play around with this stackblitz example: https://stackblitz.com/edit/ionic-4-template-cknhhj and if you have a particular question or a use case in which the gif does not work - please ask that new question providing full context.