Yes. Uninstall GS2.5 first, although I seem to remember that installing GS3 gives you the option to uninstall GS2.5 automatically.
You will want to update to v3.04. It's been clean for most people, though a small number have had better luck with 3.00.
A word of warning: You have a 10-day grace period before registering. When you do decide to register, be VERY careful which buttons you click. One button lets you restore the registration from a file. Do not click it, if the product is not yet registered! It will end your grace period and require an uninstall to get everything back to square one. And then you can't use the program until you receive the reg key from Tascam.
This isn't a bug so much as it's a usability problem. You will see the word "register" on a prominent button and will want to click it. The programmers didn't take that behavior into account.