Both players belong to my favourites, Zabaleta was one of first tennis players I've ever seen on TV (in the 90s) and Monfils, I supported him in junior time because he's different to other guys, and of course witnessed his only ATP victory, with tennis at his prime.
Both were really outstanding juniors with a great career predicted. Mariano reached his career high at age of 22 (21 ATP), and Gael - at the age of 20 (23 ATP). Both keep suffering from injuries, and both have rather energy-taking styles of game.
(Both have similar volley skills
The career of Zabaleta wasn't (isn't - he's still in action) that bad, he made USO quaters (Gael can reach it at Roland Garros - had the 4th round last year), became a Bastad king (Gael can become a king of Sopot).
Knowing Monfils' cheerful personality, he'd be the perfect person to lead the French version of "Tenis Pro" (by Zabaleta and Chela). Actually, when interviewing Mariano in 2006, I asked him who out of other players would be the best for such a programme, and "Monfils" was what he said.
Now let's expect Gael to slide more, hopefully not down the ranks, and maybe somebody gives him a hint to play same shots, but a bit shorter, but also standing 5 metres to the front.