How to subtract a shape from another one grabbing gradient continuity?

[joaosp]

I wanna "cut" a triangle (see on the image, the black one) from that rectangle's left bottom corner, but when I use pathfinder or shape builder tools to do that, the triangle "assumes" the rectangle's full gradient, and instead, I need the triangle to "grab" gradient continuity, like if it had been cut with a scissor (on photoshop I just would have to rasterize the rectangle). I wanna put that triangle on my artwork left bottom, and sure, keeping it editable.