Leveling is quite fast, especially compared to games with MS. It also helps that you get free leveling "boosters" that give 3x bonus when you hit certain levels (you can opt to recharge these through the cash shop).
Gunners start out a bit slow until they hit 2nd job. After that they're one of the fastest levelers in the game. Good damage, fast skill casts and cool times = skill spam! You'll love Shooting Spree especially when you can OHKO 7-10 mobs around you in one press of a button. 8D
Like most ranged classes, lions can't wear shields (= less equipment for defense & HP). However, as a sense type you'll have plenty of luck so you'll be able to block a good number of physical attacks as well as evade magic attacks.
Not all those who wander are lost.