Google’s grammar checker has been way too aggressive with this phrase for at least two years. The algorithm assumes ‘another thing coming’ is always right, but it completely ignores that ‘another think coming’ is legitimate and has historical precedent in English. I’ve hit this same problem writing emails with idioms that don’t follow standard grammar patterns. The automated system can’t tell the difference between real errors and intentional style choices. There’s no easy way to report false positives through Gmail’s interface either. Just dismiss these suggestions when you’re confident in your usage - the grammar checker isn’t smart enough to handle English idioms and colloquialisms properly.