If your strength is Java development you should
focus on that and find someone else to do the graphics and sounds. You can spend a lifetime learning to create decent graphics ("Learning Java in 24 hours" doesn't work. "Becoming DaVinci in 7 days" is not more probable). Even if you learn the tools (which is a challenge with something as ... interesting
as Gimp) you probably will produce "developer style graphics" - enough for the prototype and development, not good enough for production. So in effect a waste of time.
Either you buy ready-made models and graphics, you hire someone to do them for you, you team up with a graphics artist or you try to find "free" (cost and an acceptable license) graphics somewhere (just a random pick: On Turbosquid.com there are a lot of free 3D models, if you need 3D or want to render 2D - THAT is not that hard as creating the whole graphic yourself).
Graphic artist are cheaper than developers. So, easy calculation - would it be a good idea to spend your more valuable work time to learn their job than to buy a service from one without paying for their learning time (well, in small fraction you do, but not completely as you do if you learn yourself)?