Are גינה and גן entirely interchangeable? What's the difference?
The word "גינה" is used more for small gardens, like your house's garden or a small garden in which you grow vegetables ("גינת ירק"). The word "גן" usually refers to a bigger garden and is also used for kindergarten ("גן ילדים").
גינה is more the British concept of garden right? For Americans a garden isn't a yard, but for Brits it's both.
Would "I love this garden so much" be entirely wrong?
No it is ok. But they cannot have all possible correct translations in their database. So we must stick to rather literal translations as we learn hebrew and not english
then why'd you leave?