I have to agree with what most people are saying, Gastraphetes can be deadly, but slaughtered by the easiest things.
However, some Gastraphetes tactics can be pulled off, by tricking your opponet, or just toying with his mentality. Since Gastraphetes can fire at a tower without being shot back,(Pretty sure they can) you can begin chisling away at the opponets. Plus they are useful on a bunch of modes on acropolis. If your opponet has not built up outside his acropolis, fire at the economy, villagers, or buildings hanging at the edge, even if your gasts die doing that, you should have only brought a few, it is not a huge loss. Gastraphetes can also disrupt gold sites, and defend previous ones well. If you see your units cant defend against the units the Gastrphetes see, it gives you a few more seconds to make your villagers run, and to delete your storehouse/mining camp, that way, they may not set up towers there, because you may have never been there. If disrupting, just send a gast/toxote and attack a gold site, if units are there, run! Then swoop in with a bunch of Gastraphetes,and disrupt the economy as much as possible.
Besides, most of the people on ESO are acting so dumb, weird or sick, they can have no clue whats going on in the game.... O_o
^The Set Ascendant^