Alright, let's break this down.
Pre-and after the First Timeskip, Lucario just stomps. Statistically, there is no hope for Gajeel here. Lucario's also far faster than any form of Gajeel, timeskip or not. There's nothing Gajeel can do here.
Now ISDM mode Gajeel has far better chances. While still outsped, he has superior AP and Durability, although not by too much. However, his power aresenal is still smaller than that of Lucario, and his range is inferior.
Furthermore, Lucario is resistant to attacks that use metal (assuming such a technique is Steel Type, as it would be in Pokémon) and harms opponents made of metal more (assuming said foes are Steel Type). A major power here is Lucario's telepathy, which will allow it to learn Gajeel's strengths and weaknesses. This will be a hard battle, but I favor Lucario, barely.
Finally, we reach Current Gajeel. Here, the tables are turned. While again, Gajeel's power pool is still lower, he is now far more durable and lethal than Lucario, and only a few hits will bring it down here. Lucario's only hope is to stay at range and bombard with Aura Spheres, which it can do, but I doubt it. Here, Gajeel wins with low/mid difficulty.
Lucario wins, 3/4 times.