I don't see why it's unprofessional at all. Surely the whole point of open source is that you're putting it out there in the hope that other developers will take it further. If GS don't want you to re-use, why did they release it as Open Source instead of black boxing it and selling it under license?

I also don't really understand how you call anyone else "competition" in the Open Source arena. Open source isn't meant to be about competition, it's about collaboration.

From a legal standpoint you should be fine as long as you adhere to any license conditions. From a moral standpoint I think you're fine as long as you're not using the software in a way that the original devs would object to, so maybe contact them and ask if they're happy for you to re-use it unless you're sure they wouldn't mind.

I think there are a couple of arguments against what you're doing, e.g. Dilittante's maintainability issue, but they're about practicality rather than morality.

Also, consider what your goal is. Is it to produce some working functionality or is it to hone/prove your skills? If it's the former then re-using someone else's work is fine as long as they're happy for you to. If it's about honing or proving your skills then, of course, it makes sense to do it yourself.