Hi CT, and welcome to the forum. As you may know, potting soil isn't normally used in-ground; it's composition is very different from garden soil, and is designed for the watering, drainage and aeration needs of potted plants.
The only time I've used potting soil in-ground is when I had run out of peat moss, and used potting soil as a substitute since it's mostly peat moss. But that was only when I was replacing the native soil dug out of a rose hole with my usual mix of 1/3rd native soil, 1/3rd composted manure, and 1/3rd peat moss (or in this instance, potting soil).
I suspect that you will find that the replacement of 50% of your native soil with potting soil will significantly increase the rate of drainage, especially if you have gravel in the bottom of the hole. So I would very carefully monitor the moisture of the soil this summer, especially during the hottest days. Note, too, that you will probably see some settling of the soil, which may cause your rose to "sink" a little bit. Some of the settling will occur from the loose soil finding its way into the air pockets and nooks and crannies of the gravel after being watered, and some settling may also occur through natural compaction. If this happens, you can top it off with compost and/or mulch. In fact, be sure to mulch now if you haven't already, to help keep the soil moist and cool, and to suppress weeds.
