Pamela's answer worked perfectly for me, but I can see in comments there is some confusion so here it is edited. I don't have enough reputation to add this as a reply hence new answer...
Open the document in GoogleDocs/GoogleSheets app on your iPhone.
Tap the three dots in the top corner Share & export Copy link
Leave the GoogleDocs/GoogleSheets app
open Shortcuts app
Use the search box and type "Safari", scroll down to see 'Open URLs' and tap
Tap the little gray field that says URL Type googledocs://
Tap and paste (the link you copied from GoogleDocs). [If you are using GoogleSheets type googlesheets:// instead.]
The result will look something like http://googledocs://https://docs.google.com/document/d/bunch-of-random-characters
Tap the '...' next to where it says “New Shortcut” at the top in big bold letters and name the shortcut and you can change the icon (the gray square with the magic wand in it) to what you want. Tap done.
Tap the 'share' icon at bottom of screen and then Add to Home Screen Add Done Done/Next
Voila! You now have an icon on your home screen for the document, which will automatically open in GoogleDocs or GoogleSheets.