Yeah, 18x10 with a +15 offset will bring the rims out 27.7mm so roughly an inch.
I personally think a 9" wheel on a GTR would be too small. They come in 9" wide so I would go 10" at least.
So for a 10" wheel, +15mm offset will bring the wheel out 27.7mm.
10.5" with +15 will bring it out 34.1mm.
Just to give you a little guide, to get a 9" wheel to come out an inch, the offset needs to be +5.
For it to come out ~1.5", it'll need to be -7.
With a 9.5" wheel, for it to come out 1", the offset needs to be +11.
To come out 1.5", it needs to be -1.
10" wheel to come out 1", it needs to be +17.
To come out 1.5", it needs to be +5.
10.5" wheel to come out 1", needs to have an offset of +24.
Out 1.5", offset needs to be +12mm.
You can use that guide when looking at wheels. Hope that helps.
Cheers