This is a tricky question because we have to define "productive." Gabbert's career has been productive to his bank account. He's about to enter his ninth year in the NFL. According to Spotrac.com, his career earnings are close to $20 million. As long as he's been smart with his money, and knowing Blaine and his family I assume he has, he is set for life. Ninety-nine point nine percent of Americans would call that a productive career.
But on the football field, obviously he's come up short in establishing himself as a successful starting quarterback. He's thrown 48 touchdowns to 47 interceptions. He has started 48 games over his career for four different franchises and is now on his fifth team, as the backup in Tampa Bay. His career numbers aren't great. But he's apparently got enough tools to keep steady employment in the league.
Eight years from now will Lock still be in the league and putting up better numbers than Gabbert as a starter? No guarantee that happens, but I'm leaning toward yes. So much depends on timing. If Gabbert had gone to a better franchise than Jacksonville and wasn't forced to start to early maybe he develops better and has a better career. Denver is a good spot for Lock. No pressure to play right away. The defense will be sound. They've got some good young pieces around him in Lindsay, Sutton and Fant. He's got all the tools to have success in the NFL, and if things go right and he develops at the right pace, more success than Gabbert.