Patchwork Gnomes can regenerate even if it isn't in combat, it's already tapped, or it's undamaged.

Activating the ability creates a replacement effect that acts like a shield, replacing the next time Patchwork Gnomes would be destroyed that turn. This shield works against effects that try to destroy Patchwork Gnomes or lethal damage that would be dealt to Patchwork Gnomes.

You can activate the regeneration ability even if Patchwork Gnomes isn't at risk of being destroyed. Sometimes you need to discard cards. We understand.