Where's the auto-replace function of the keyboard
This functionality is covered by the operation of the Personal Dictionary. Settings > Language and input > Google Keyboard > Text correction > Personal Dictionary. Select the language to which you would like to make the addition.

Finally, the '+ Add' button allows you to define a word or phrase, and, optionally a shortcut.

So for your example, Word = 'As soon as possible', shortcut = asap.