Anchoring Gazebo in planter pot
To answer your question, yes, concrete in a planter would most likely crack it due to its expansion and contraction caused by changes in humidity and temperature. However, a plastic 5-gallon bucket will hold concrete, and you could camouflage the bucket by putting it in a planter and covering it with dirt. Depending on where you live, four 5-gallon buckets of concrete might not be enough to guarantee your gazebo won't be moved by wind (thereby bending and destroying it.) You could consider using half wine barrels filled with concrete instead. I would expect 4 of those to be an adequate anchor.