Can I use the Apple Watch 7 when I leave my iPhone home?

It appears you already own Apple Watch 7, is that correct? If it is a Cellular model and you have set up cellular service on the watch with your cellular provider, it will work away from your iPhone. If however, you own a GPS only model, it will have limited functionality away from your iPhone.

Yes, but you may experience limited functionality, especially when not on Wi-Fi. It is advised to purchase a cellular model of Apple Watch, if you intend on using it a lot without your phone nearby.