How to disable rotation during zooming in the Google Maps app?
If I open the app, and make sure I first make a pinch movement with my fingers, it doesn't rotate the map when I rotate my fingers in the same movement afterwards.
So if you want to only zoom, it is best to first make a little pinch movement, to let the app know you're going to zoom.

If you on the other hand rotate your fingers first, it will start rotating the map.

If you only want to zoom in, you can also use the double tap option. That will not change anything to the rotation. But this is a little more work if you want to zoom in a lot.