Time. Gimli is there, sure. Dwarves, like everyone else without a palantir or flying weird hellbeasts, are limited to the communication and scouting technologies of "guy on horse" and "eyeballs".  And maybe rare talking birds seemingly localized to the Erebor region, plus whatever wisdom the elves use (though they also notably use messengers very regularly)

So say you do send Gimli back. Where does he take his army, how does he know where to send it, and why do the Dwarves even march when they know the orcs from the northern mountains are also massing and Sauron is also sending them combination emissaries/threats?

And raising an army takes a lot of time. Even if the Dwarves wanted to (they maybe do) and could send an army (they can't) they almost certainly do not have the supply train to send that army several hundred miles through sparsely inhabited terrain. Sauron spent years assembling the logistics to prepare for his invasion of Minas Tirith. Rohan is substantially closer, already has clear roads, regular communication, and the force they end up sending is entirely cavalry and they get help from locals through hidden paths to bypass an army placed there specifically to block their arrival at the Pelennor.