Gnorc Gnexus is quite a small world and not all the gems are available when you first get here. You will have to finish 2 other worlds before you can get all the Gems here. Pick up what gems you can (75) and release the one dragon right in the middle.
As you can see there are 3 huge Dragon heads, one portal and the balloonist. Since the Dragon heads are closed, and you don't want to go back, you might as well take the one open portal to Gnorc Cove.
Part 2 - get to end of Gnorc Cove
When you come back from Gnorc Cove the orange Dragon head will open to reveal the portal to Twilight Harbour and a chest with 50 Gems.
Part 3 - get to end of Twilight Harbor
As you return you see another Dragon head open, this time the purple one and now you can face Gnasty Grorc. There is also a chest with the last 75 gems in it.Part 4 - Defeat Gnasty Gnorc and watch credits
That was fun. Now I bet you were expecting that last Dragon head to be open. Sorry, it's not. I wonder what happened? Let's ask that Dragon.
Magnus tells you that in order to open the green dragon head you need to collect every gem (12000), free every Dragon (80), and save all the Dragon eggs (12). If you got everything along the way and Magnus was your last Dragon, when he's done talking the green Dragon head opens and you have access to Gnasty's Loot. If not the you better go back and get them.
You should now check your inventory. Make sure all levels are listed. There should be 6 levels in each of the fist 5 worlds and 4 in Gnasty's World. Then look and see what level you are missing stuff in.
There is one other thing. If you have everything and the Dragon head still doesn't open there is one thing you forgot to do. You probably forgot to use the return home whirlwind in at least one level. You should now go back to each level and make sure to use the Return Home whirlwind.