On your iPhone or iPad go to Settings > Accounts & Passwords > Add Account > Other.

Tap on Add Subscribed Calendar.

In the Server section, copy and paste this link.
https://calendar.google.com/calendar/ical/hes25avaku24kij1ju2g5q8co4%40group.calendar.google.com/public/basic.ics ou

Next you can choose a Description.

Finally, tap the Use SSL switch, and hit the Save button.