Has the gravity a negative value? We all know that the acceleration of the gravity is equivalent to 9.81 m/s². But the gravity attract everything that have a mass (everything as far as i know) to the center of a planet.
As an magnetic field that attract another magnetic with a opposite pole, is the gravity an negative value to attract the positive masses that have in the planet (specially the earth)?
 A: Gravity is not “a negative value”. Gravitational force and gravitational accelerations are vectors whose direction is attractive. What this means is that, if you have two massive objects, each one experiences a force towards, and accelerates towards, the other one.
Two masses attract even though both masses are positive. (As far as we know, all mass is positive.) By contrast, two positive charges repel. To get the vector pointing in the attractive direction, there is a negative sign in the gravitational force equation. But this does not mean there is a negative value. 
A: A gravitational force is a vector quantity.  A vector is never negative. It has a positive magnitude and a direction. After you define a coordinate system, a vector can be broken into components, some of which may be negative.
A: Outside involved masses gravitational energy potential is ~ -1/r for distance r; in this sense YES. The offset of the absolute energy of a system does not contribute to the laws; regarding this neither YES nor NO.
