I really never considered the Gap of Dunloe part of Killarney National Park, even though geographically it probably is.
To me Muckross House/Gardens, Ross Castle and Torc Waterfall seem to be more what I would consider Killarney National Park. From the town centre of Killarney, Muckross and Ross Castle etc. are in opposite directions from The Gap of Dunloe. However, when you do the Gap day long tour, you end up at Ross Castle after crossing the lake. So I guess its all part of the package called Killarney National Park.
In my opinion a day trip to the Dingle Peninsula is far better than the Ring of Kerry. So if you have to choose one, I think most would agree, do the Dingle Peninsula.
If you do skip the ROK at least drive up to Ladies View for some great scenery. Just keep driving past Muckross House and up the mountain road. Also, be sure to do the Ross Castle tour (arrive before 4 PM) It will be to late to do the tour after the Gap day tour.
We did both the Gap and Dingle tours with Deros and were pleased.
The Gap of Dunloe borders portions of KNP but is not considered part of it.
If you are interested in seeing how the Irish lived on farms in the early 20th century you might enjoy Muckross Traditional Farms. Also Muckross Abbey, Cherry Tree Walk, the view from Aghadoe.
The ROK is different from Dingle. Not necessarily better or worse. Both have good ocean views, beaches and mountains. Dingle is smaller and some say it gives more "bang for your buck" especially Slea Head Drive. Depends on your interests. Both can be done as day trips from Killarney. Will you have to choose only one?