PGraphics pg;

void setup() {
  size(100, 100);
  pg = createGraphics(100, 100, P3D); // make your buffered image size equal to sketch size
  makebackground();
}

void draw() {
  background(pg);
}

public void makebackground() {
  pg.beginDraw();
  // do your gradient stuff here
  pg.background(102);  // you can do background statements in the background
  pg.stroke(255);
  pg.line(40, 40, mouseX, mouseY);
  // after your gradient, close out the pg image and finalize it with pg.endDraw
  pg.endDraw(); 
}

1. void draw() {
2.   //set fill to gradient
3.   rect(0,0,width,height);
4. }