If your plane goes straight up or straight down, will the GPS record a correct speed?

What I'm getting to, is if the plane is in a deep dive, it is travelling only partly in the horizontal plane, and partly in the vertical plane. Even at 30 degrees from horizontal going up or down... The GPS might not be as precise in the vertical plane, or not even record a change in distance, depending upon the triangulation of the satellites. If true, any significant flight angle would also give a false high speed reading due to lack of resolution or sensitivity vertically.