Are you defining any Network Masks when you configure Google with Okta?
If masks are not defined (blank) then any new device enrollment (adding gmail account) will be redirected to Okta for authentication (first time). Devices that are already configured with gmail will not be impacted.
If you have netmasks defined then for the defined netmasks it is same as above, for all other networks Okta doesnt come into play.
Keep in mind, once you integrate Google with Okta/SAML, none of the users can change their Google passwords (network mask rules doesnt apply here). They may need help for their admins in case if they need to change it for whatever reasons. You other option is the one that Raja mentioned above.