Question: how did the earth gain gravity at first place?

Mark Kennedy answered on 14 Nov 2016:

Gravity has always been around. It’s an inherent trait of what we call “space-time”, which is really complicated but awesome concept.

Initially, the Earth and our Solar system would have started out as a disk of debris and dust orbiting the Sun (here’s an actual image of what one of these disks looks like: ).

As little bits of dust and debris begin to clump together, their gravitational attraction increases (more mass focused in a smaller area increases your gravitational energy). This increase in gravitational attraction causes even more dust and debris to accumulate, until you get a spherical object which has sucked up all of the debris in it’s path (that’s what those dark rings in the image I linked are – planets that formed and cleared out areas of the disk).

So the Earth “gained” it’s gravity when it became big enough to attract all of that dust in its path and form a planet. Actually, one of the rules for being classified as a planet is you have to be big enough to hold yourself together as a spherical object (which is why comets don’t count as planets) and that you swept your orbit clean of all large pieces of debris (which is one of the reasons why Pluto isn’t a planet, because it hasn’t cleared it’s path)