is there such thing as zero gravity?

Only if there is no mass. But, there is mass, so... no. We can come up with technology that helps us get around gravity's influence (like planes and helicopters, etc.), but gravity will be present anywhere there is mass. Another point is that, even you get really really really far away from an object with mass, gravity will be very weak, but still present.

What used to be called zero-g is now more commonly called microgravity, because of the realization that zero-g isn't attainable. But zero mass isn't necessary; freefall is a microgravity scenario, so it includes things in orbit. This is because freefall is indistinguishable from no gravity whatsoever, by the equivalence principle.

What used to be called zero-g is now more commonly called microgravity, because of the realization that zero-g isn't attainable. But zero mass isn't necessary; freefall is a microgravity scenario, so it includes things in orbit. This is because freefall is indistinguishable from no gravity whatsoever, by the equivalence principle.

 

I realized that freefall was not the same as zero-g, so didn't bother mentioning that. However, I didn't realize that the equivalence principle worked both ways. I knew that, for example, riding upward in an elevator at a certain rate was equivalent to gravity due to the acceleration, but was unaware that the same idea applied also to the indistinguishability of microgravity from no gravity... So, thanks for that. A new feather in my cap you've given.

Gravity and acceleration are indistinguishable, so it really has to work both ways. If you are accelerating you can't tell if it's gravity, and if you aren't you don't know if it's no gravity or being in freefall.

 

It's GR, though, so the details can get complicated very quickly.