I'm pretty sure that I was able to search gifs in native keyboard app on my Galaxy S8 a month ago or so. Now checking and there is lack of search button while GIF tab opened.
Have seen a topic in S9 thread here where guy stated that GIF search has been fixed in December update, but I do not see that though phone been updated till now.
How to search gifs in stock Samsung keyboard app? (for now there are only predefined categories to chose from)
**Galaxy S7. Stock Messages. Samsung Keyboard V 184.108.40.206. Android 8.0.0 Samsung Experience 9.0
Two options to search gifs: FYI: You can do both.
1. Download Gboard from the Play Store. Set Gboard as your default keyboard. Gboard has a built in gif search button "GIF".
2. Download imgur app from Play Store.
a. Open Messages. Open a new or old message. Tap the paper clip icon left of where you enter message. Tap "Image". Select imgur Reaction GIFs. Search away. Next time you tap image, it automatically opens imgur gif search.