No, your sealed, aka gel-cell battery won't vent, unless you seriously overcharge it and pop the safety pressure release. That, and their low cost compared to NiCd or NiMH, are why they're used for things like emergency lighting and similar indoor applications.

If the electronics tech says you can use 20 gauge for a single fan, and 16 or 14 for the combined load, then any larger wire size (smaller number) will also work. As suggested, if you're going to be drawing 3 amps through the main wire (1.5 A times two fans, wired in parallel), you should certainly have something heavier than the 24 ga. that the fans use for their final pigtails. You should also include a fuse in series with the combined load, preferably about a half amp higher rating than what the fans will draw (so in this case, about 3.5 amp) -- that way, if something shorts, it'll blow the fuse instead of burning the insulation off the wires (a car battery can melt 4 gauge jumper cables, and even a gel-cell is capable of delivering more than enough current to melt pretty heavy copper wire, though your solder joints, if used, will melt first -- but the insulation will burn before solder melts, and it's pretty stinky stuff).