To get the selected Caltrics calendar events into the Google calendar app (recommended) on your Android phone or tablet, you first need to add your Caltrics subscription to Goggle calendar in a browser (not the App) using your Google email address.
(If you are unaware of your Google email address, see (How to find the Google email address associated with your Android phone or tablet))
Follow the procedure detailed here How to manually add a Caltrics account calendar to Google Calendar
Then open the Google calendar app on your Android phone and click on the 3 vertical lines and select ‘Refresh’. Then click on the menu (hamburger icon) and scroll down to your Google email address, under which, Caltrics will be listed. Ensure that this is selected (ticked).
If Caltrics is not listed, scroll down and click on ‘Settings’ and under your Google email address you will see Caltrics (may need to click on ‘Show more’). Click on Caltrics and change Sync slider to ‘on’ (You can also change the Caltrics calendar colour here).