Wait! My Pet is my Girlfriend?!

Official Title: I Was Going To Kill Myself But Then I Fell Into a Gate and The System Gave Me a Pet as My Girlfriend
私は自分を殺そうとしていたのですが、それから私は門に落ちました、そしてシステムは私のガールフレンドとして私にペットを与えました
Watashi wa jibun o korosou to shite ita nodesuga, sorekara watashi wa mon ni ochimashita, soshite shisutemu wa watashi no gārufurendo to shite watashi ni petto o ataemashita
(Japanese title translation just for the lols)

21 year old guy had the good life, university, a good part time job, and the best girlfriend he could ever ask for.

Until one day gates appeared all over the world, releasing monsters into reality. Humans seemingly having no choice but to surrender, no power to fight back until someone figured out going into the gate turns them into a [Player]. Now the world welcomes its everyday heroes, guilds, and ranked players... All of this our protagonist wants nothing a part of.

It's nothing to live for when his girlfriend is gone. That is until he stumbles into a gate himself, leveling up game-style with a system default growth type pet that turned out to be... His girlfriend?