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For your first game your ambitions are maybe too high. Start a lot smaller with a small linear level and then get bigger. I would give you this tipp with every engine.
But to your questions:
1) Open world is possible, but the world is small and needs to be optimized a lot. There is no level streaming or anything similar in GameGuru at the moment, so do not expect a Skyrim map.
2) You can change the sky, the fog and the lightning, so I would say yes it is possible and if you search in the scripting forum you will find some scripts which needs to be adjusted. But there is no kind of moving sun, you can fake it or modify the GG source.
3) If you use the right format of models and mind the modeling conventions GG have, of course. (I am a coder, so I am not the best one to answer this)
4) Yes, you can. Here is the github page: https://github.com/TheGameCreators/GameGuruRepo
You can not create your own engine with this, but you can modify the engine and if it is a worthy addition, you can give it to Lee (main dev) and he will add it to the source. The normal github way(create a branch, pull request and so on)(Read the GameGuru source license!
Quote: "Import such things as havok (example lol)"
You can try, but with 5 months of experience, it is maybe too hard. 3D/physics programming is another world. OH and it is C++. So if you are a java programmer, you will need a lot of things to learn.
5) A quest log is possible. The best way is to code it in lua(learning lua is always the best way to start into GG, the engine use it and it is never wrong to learn a scripting language)
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