Answer:
1. some ways I might first observe gravity are... when I first arrive on earth I will see everything staying in one place on the ground and nothing will be floating
everywhere.
2. I might test gravitional force by jumping and seeing how fast I come down or by taking something and dropping it from above my head to the ground to see that it falls and doesn't float.
3.I woukd describe gravity to other aliens back at my home planet by #1 telling them that nothing floats, everything touches the ground and if u drop something from high above you will instantly/immediately fall back down to the ground.
You can say exactly this if you want bc I answered like how if u were gonna answer the question so your welcome