gpu_mem applies only to the propietary Broadcom driver, emteria.OS is using the upstream Gallium VC4 driver. The "cma-256" in the config.txt is what sets the memory for this driver.
Feel free to try different values, but I highly recommend you leave it at the maximum 256 MiB. Android isn't particularly aggressive in releasing GPU memory and when you run out, the system will simply freeze.
We're working on a system that monitors GPU memory and reclaims from background activities, but it's not there just yet.