Google introduced a new version (6.5) of the Gboard Beta that now supports sticker packs and Bitmoji directly on the virtual keyboard.
The novelty, which can be found by touching the smiley button, displays a new tab next to the GIF button, with the same sticker icon previously seen on Google Allo.
You can then install sticker packs and compatible apps on Google Play; and also use the popular Bitmoji service.
You can install the Beta version of Gborad from this link. It is possible that the new features will disappear at any time.