Follow these instructions to use the Emoji Kitchen:
Tap the text bar to bring up Gboard.
Tap the emoji icon on the left side of the space bar.
Select an icon as your “basic” emoji.
Tap the second emoji you want to add.
Choose one of the fun mash-up options right below the text bar.
In the Emoji Kitchen, Google is cooking hilarious fusions
Please note that gesture, person, and flag emojis are not supported for use by the Emoji Kitchen. One suspicion of the reason behind this limitation is that a merger with emojis from those three categories could be disrespectful and/or offensive to certain people or a certain country.
Tweet from Emojipedia discusses what is cooked in the Emoji Kitchen