Hi there

Here are my settings for GPU rendering

Grainy renders,this depends on more factors,how many lights are you using and what materials are you using

I would try have look on those settings,but I found,on some earlier GPU like GTX780 you can use 256 bucket size and on newer like is GTX970 and others you should try to use 128(192 bucket size I've used as biggest without penalty on speed and quality),usually all my renders are up to 50 samples,some interior needs to baked up to 70 samples

Hope this helps

Thanks,Jura