To restore Google Calendar events or CalenGoo settings you have to either restore a backup created with CalenGoo under "Settings > Backups…" in CalenGoo or you can restore a backup of your iPhone/iPad:

iTunes Backup

Each time you sync your iPhone/iPad with iTunes on your computer, iTunes creates a backup. However it keeps only the most recent backup, i.e. you must not sync again after you have lost events. Instead follow these instructions:

1. Restore your iPhone/iPad by following Apple’s instructions here: https://support.apple.com/HT204184
2. Turn the "Airplane mode" of your iPhone/iPad on to prevent it from syncing.
3. Export your calendars as ICS files and email them to yourself (still while in "Airplane mode", they will be sent later): ICS export
4. Turn the "Airplane mode" off so that your emails are sent. Then import the ICS files into Google Calendar as explained on the website in step #3.
5. After everything looks correct in Google Calendar you can tap "Settings > Reset calendars" in CalenGoo to erase all events from CalenGoo’s database and re-download everything from Google Calendar.

iCloud Backup

If you have turned the iCloud Backup of your iPhone/iPad on, a backup of your iPhone/iPad will be created at a regular interval and saved into the iCloud. You can restore the backup ( https://support.apple.com/HT204184 ) and then follow the instructions starting from step #2 in the section above.

Apple automatically creates backups of your iCloud calendars at a regular interval (currently daily for about a week). You can restore them by following these steps:

1. Sign in on https://www.icloud.com
2. Open "Settings".
3. Click on "Restore Calendars and Reminders" in the lower left corner.
   
   
   
4. Choose a backup to restore