MarekVit already posted a very good solution and there is not much i have to add, but i figured something out that should speed up the whole progress. As you probably know, you can't trigger a gnome if you leave the pvp match with the retreat option in your menu. You have to win or lose the pvp match to "roll the dice" for a new chance. You can use this knowledge to your advantage. How?
Well, if you leave a gnome pvp fight with your menu, your not "rerolling the dice" and the next pvp fight is another gnome fight. If you met a treasure or pet gnome is randomly chosen at the start of the fight, but the chance to meet a treasure gnome is much higher. With the possibilty to respawn the gnome, you can leave and reenter a pvp fight until you find a pet gnome. With other words you can change/transform your treasure gnome to a pet gnome.
This method probably get patched in the future. Keep in mind giving this method a shot could end up with losing your treasure gnome reward in worst case.
ScuzzyBunny posted another method in the comment section, it's nearly the same thing, but he suggest to leave the match via dashboard.
Casual pvp, when a treasure gnome spawns press guide button and close game (DO NOT PAUSE AND "RETREAT")...launch game again, choose casual pvp, and the treasure gnome will be there again (or it will have switched to a pet gnome). If its a pet gnome play it out (ensuring you win and dont let him escape), if the treasure gnome is there again, guide, close game, relaunch, and repeat until you get the pet gnome. I did it today and I got the pet gnome on the 3rd launch...but getting the gnome to show up at all took about 50 pvp games.