Depends on your Google Photos app version. As of v1.23, deleting local gallery photos should prompt you -- Read more about it here
As quoted in the article --
Say you took a few photos on your phone and they're already backed up
to Google Photos. Then you went through another gallery app (if you
use the Samsung or LG or HTC Gallery apps or another third-party
client) and deleted the ones you didn't want to keep. The images would
still be backed up in your Google Photos account.
Now, Photos is smart to realize that this has happened and suggests it
removes them too. For example, you may want to do that for burst
photos that you don't want in your account or for random shots that
you only needed for a couple of days. But you don't want to do that
for photos that you want to save but just removed because you're
freeing up space on your device. That's why it's great that this is an
optional card and not an automatic action.
You've set up google photos to backup over wifi and when charging. So the next day you delete the photos via a different gallery app, google does not have an idea that you've actually deleted them locally.
If you want to delete local copy while retaining the cloud copy, continue what you're doing now.
If you want to delete cloud + local copy, one of many ways would be to delete it from the photos app.
- Open photos
- Select the photos you would like to delete
- Tap on the 3 dot overflow menu on the top right of the screen
- Select "delete device copy"
- Tap on the bin icon at the top to delete the cloud copy as well.