I can only presume the earlier arrivals from Greece, etc. had been eaten for dinner by the fierce Celtic warriors who controlled the heartland.
Be interesting to do some DNA studies to see which Turks are what ~ right? Odds are good that the Celts are still in there.
The Celts did invade Anatolia and gave the name Galatia to the area where they settled.
But they also invaded Greece, Italy, and parts of Egypt. The Goths invaded Italy and Spain. But in all these instances, I don't think there was any massive impact on the native popualtion bases there from a genetic perspective.
I guess the point is HOW many were part of the invasion force, and what was the composition of that force.
If the invasion force was massive, you could expect a significant impact on the population. If it was mainly warrior raiding parties, perhaps not. If the invading force include families, again, the impact would be more significant.
When you look at genetic markers, parts of Anatolia are distinct from most parts of Europe - with the interesting exception of Tuscany. It has been postulate that the Etruscans may have come from ancient Anatolia.
Before the Celts arrived, there were large Greek and Persian populations in Anatolia, and before them a very large population base of Carians, Lydian, Lycians, Hittites, Luwians and other people like probably Assyrians in Eastern and Southern Anatolia. I think these groups contributed more significantly to the population base than the Celts.
As for the actual RACIAL impact of the Seljuk and Ottoman Turks, I think that would make an interesting Ph.D. thesis in population genetics.