A lot, apparently.
Gilbert is a 30 year old QB who has been out college since 2014, thrown 44 passes in the NFL and only completed 52% of them. Anything below 60% is trash in today's NFL. Six other NFL teams cut Gilbert before Dallas signed him out of desperation.
Even in his one legendary start he only completed 55% of this passes. His passer rating for that game was 72.6 and it's 70.5 for his career. Dalton completed 64% of his passes last year and had a passer rating of 87.3, so Gilbert was only better in your imagination.
He is not "better than most backups" when you consider you're talking about guys who have started - and won - several games in their NFL careers. Guys like Mariota, Tyrod Taylor, Teddy Bridgewater, Nick Foles, Blake Bortles, Joe Flacco, Blaine Gabbert, Colt McCoy, Mike Glennon, Mitch Trubisky, Jacoby Brissett, and Gardner Minshew. Gilbert isn't remotely better than any of those guys. He's a one-start, seven-year scrub still searching for his first win.