Without GPS you can use altitude control, but it will drift horizontally and require human input at all times. That’s unless you add some sort of position or velocity estimate from other sources. This could be a tracking system like VICON or OptiTrack, or onboard using vision (SLAM, VIO) or optical flow. Using a flow sensor is probably the cheapest/easiest but it will still have drift over time as it’s only giving you a velocity estimate and no position as such.
You should be able to try it in SITL, I think there is a model called iris_flow, then you’d just have to disable GPS. You can use failure injection to set GPS off, or you can just comment out this line to prevent it from publishing GPS: