@jorgk: I tested on a macbook pro + Mavericks and the test with a fixed duration works fine (with the GeForce or the Intel GPU).
Maybe it's related to a recent update? What is your version of OSX?
I will release a new update of GpuTest shortly, maybe that will fix that mysterious bug.
I'm on Mavericks, 10.9.5.
Actually, it went away after a restart (I mean, the non-quitting-GPUTest did not happen then, always quitted just fine).
However, it reappeared again (and I am too lazy to restart). Not sure what it is: The progress bar reaches it's ends, the GPU already cools down, but the application stays on, just as if stalled.