Am I finished?
I've been doing the Scottish Gaelic course and I skipped ahead to the level four skills test. I completed it and got the following message... "First of all, you're awesome! You've completed the Scottish Gaelic skill tree! Second of all, we'd like to take this time to thank you sincerely for your lengthy participation on Duolingo. So in honor of you, here is a very special trophy in which to show our gratitude."
Does this mean I've reached the end of the course? I couldn't see any more levels to do but I'm definitely not fluent. Do I have to go back and do all the bits I skipped over?
That message appears when you complete the tree with at least the first level. Are all your skills displayed as golden? If not, you can complete them until golden. If they are, they'll prompt you to practice them again in several days by displaying a "cracked" ring around the skill. You can always practice skills again if you want. They will be adding more skills in the future too.
This means you have cracked the Level 1 tree through shortcuts ( i.e testing out ). Now you can improve on those learned skills by doing Level 2 to 5.
But keep in mind, Duolingo won't make you fluent in any language.
It is just for elementary skills. In some languages upto a lower intermediate skillset - that too mostly in contextual vocabulary, linked grammar.
You have to use many other resources like Learngaelic.scot and interact with Gaelic speakers to become "fluent" in Gaelic
I've not completed the course yet, but usually you can go back and start again and get to level five in all of them to get a golden tree. It's not clear to me if that's what you've done - and it's possible that being brand new the course doesn't have that yet. As I said, I'm not that far ahead to tell you myself.