Replacement Theory, as promulgated almost endlessly by Fox News (especially Tucker Carlson) is quite explicit -- that other races, aided and abetted by "the Jews" are out to replace the rightful "owners" of the United States -- the white race.White supremacy is bad. Violent white supremacy is very bad. Few people will disagree with that.
That doesn't clarify what you mean by GRT though.
Opposition to excessive immigration is often because of cultural differences, not 'racial' differences. Is this GRT?
On the other hand, I've seen people complain about white people moving into traditionally minority areas and resulting gentrification. Is this GRT as it is a racial preference?
Remember what was being chanted in Charlottesville, that caused Trump to proclaim "very fine people on both sides:"
"Jews will not replace us. You will not replace us."
But in my view, maybe those folks are just projecting their own guilt, because they know full well that that is precisely what they did to the people who were the "real" Americans until Plymouth -- indigenous people's. Yes, they took their land, bundled them up to live on reservations, tried to get the "Indian" out of their children through residential school systems (we did it in Canada, too, but we're in the process of admitting it and trying to reconcile).
And they themselves (well, their ancestors) filled the nation with black people -- but only because they saw them as little different than draft animals and washing machines.
And having done all that, they've now managed to convince themselves that it could happen to them, and it terrifies them.