It sounds like what you've got is a bunch of instances in the standard coordinate system, and you want to rotate only the view while keeping the same coordinate system, and you want to know just the y value of each instance in the rotated view. If that's correct, then while I'm sure there's a really simple way to do this with matrices (and I implore someone who knows them better than I do to respond with it), I think we can handle this with trigonometry.
First, I assume that the origin that the view rotates around is not at 0,0. So let's shift everything so that it is. Assuming that you're calling this from each instance, and if not, replace the variables accordingly:
GML:
var newX = x - origin_x;
var newY = y - origin_y;
Now everything is centered with the point of view rotation at the origin. Next, we need to convert this point to polar coordinates. You can use trig for that, or GML functions:
GML:
var r = point_distance(0, 0, newX, newY);
var theta = point_direction(0, 0, newX, newY);
In polar coordinates, it's super easy to translate the coordinate space with a rotation.
And now we just need the y coordinate of the new point.
GML:
var rotatedY = lengthdir_y(r, theta);
That should get you what you need. There may be some need for (90 - theta) conversions, depending on how you're calculating your draw angles, but I hope this idea gets you to the solution you want.