I don't think we'll see the first gym being Rock-type. The only generation in which each start didn't have a super effective move against Rock by the first gym was first gen, and technically Metal Claw Charmander in FRLG counts. Naturally, the Water and Grass starters have super effective moves (usually Water Gun / Razor Leaf). Gen II we can rule out since the first gym was Flying, not Rock. Combusken has Double Kick, and Monferno has Mach Punch. Unless Pokabu evolves into Fire/Ground (which would be awesome) or Fire/Steel (also awesome, but probably too good defensively that early), I doubt they'd want to give players who choose Pokabu that large of a disadvantage (especially since there may not be any Water, Grass, Fighting, Steel, etc Pokemon that early in the game). That's my theory, at least.