The difference between 'Herzlich willkommen' and 'Willkommen' is probably clear, the first one is more polite and sounds friendlier. 'Gern geschehen' is the reply to 'Danke', when someone has helped you. Beispiel: Danke, dass du mir geholfen hast. Kein Problem, gern geschehen. Translatiion of 'gern geschehen': Don't mention it! You are welcome! My pleasure!

I don't see a big difference there. Comparing them is a bit hard, though, since they're used in quite different situations.

thank you

They are used in completely different situations. "Gern geschehen" means "you're welcome", but "herzlich willkommen" is a more friendly way of saying "welcome" in like "welcome to my home".