Personally, I don't think the Gabriel and Kos partnership is better than Merts and Kos and I wouldn't tinker with that understanding at the tail end of the season.
Check the stats and you'll see we've lost more games with the Kos and Gabriel partnership this season. We've lost 9 games across all comps this season but 4 of those games we've lost with Kos and Gabriel as CB pairings. They hardly keep clean sheets either. Best to explore that partnership next season but for I wouldn't switch things up now when they haven't been successful so far.