Native instruments Komplete Start is free and that has organs I believe. Has plenty of instruments that you may use in fact and is a very good freebie.
As for Grace, as it’s a sampler instrument you load the samples or presets via the plug in so up to you where you store them. I’d create a folder especially for them, unzip there, and you can organize how works best.
Then simply load from that location within the Plugin. If you’ve not used a sampler before it may require a little bit of learning. However, you open up to a load of free content as you can use raw sample packs which are very easy to find.