Okay you are confusing what happened in the comics to what happened in young justice. What happened in the comics was that rita managed to convince dayton to adopt gar instead of his legal guardian, galtry, who was stealing his inheritance, and stayed as his step father after rita died. Young justice essentially combined dayton and galtry together.
However,what happened in the comics was that thanks to overusing the mento helmet to help out constantine, mento became stark raving mad and tried to kill gar and the titans until raven managed to reconnect him with gar with her empath powers. However he unexplicably became bad again in the guise of the "crimelord", being obsessed with killing slade. Stupid bullshit basically.