I do not have a smart phone, it matches me for stoopid and I use it as a phone only.
I am sure there are folk here with a Whyphone who can describe the process.
I found this with a brief search.
 
Send and save GIFs in Messages on iPhone
In Messages on iPhone, use #images to send a variety of expressive images and GIFs.
Apple Support (support.apple.com)
 



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