Deleting from the Gallery (not the Google Photos app) should not delete the Google Photos copy . If the copy you see in the app also disappears then it usually means that the photo is not backed up to Google Photos. Photos actually IN Google Photos can only be seen at the Google Photos Backup location - Tap here
The safest way to clear device storage is to use Free up space by tapping your account icon top right to choose that option. The app will only remove photos after checking if they are backed up first.
Note that the photos will no longer be available to view in your gallery and might not be available to choose from Social media apps. WhatsApp currently has access, Facebook currently does not. Unsure about others, but still possible to share to apps from inside Google Photos
Photos can also be deleted by selecting them and the to use the 3 dot option to Delete from the device. This will also give a warning if the photo is not backed up.