Cross Reference: An Add On for Google Docs
Cross Reference is an open source cross referencing add-on for Google Docs that automatically numbers elements such as figures and updates and formats in-text references to them. Install.
Short instructions
Create a label for a figure by writing the word 'figure' in the caption and then adding a hyperlink to that word with the URL #figur_YOURCHOSENNAME. Refer to this figure in your text by writing 'figure' again and adding a hyperlink with #fig_YOURCHOSENNAME. Update the document via 'Add-ons -> Cross Reference -> Update document'.
This software is released under the MIT license.