I have a Kohler K-9620 whirlpool tub and I am trying to replace the grip rails
The anchor nuts have rusted and broken so there is no way to secure the screws.
Does anyone have a suggestion on how I can secure the grip rails. There is no way
To get underneath to hold an anchor nut.

You will have to get behind the tub to install screws and nuts. Installation Instructions by Kohler:

If you can't get to back side to secure a nut, you may be able to use toggle bolts.