You can use custom path for the Envoy log. Please edit the gm-config.cfg to set the new path.

Config file locations based on your OS:
WINDOWS: "C:\Users\%USERNAME%\AppData\Local\GridMarkets\Envoy\gm-config.cfg"
LINUX: "~/.local/share/GridMarkets/Envoy/gm-config.cfg"
MAC OS: "~/Library/Application\ Support/GridMarkets/Envoy/gm-config.cfg"

Changing the path:
Open the file for editing and change the "default" value for the "envoy_log_file" parameter to your desired custom log path, example:
Entry responsible for using the default log path:
"envoy_log_file": "default"
Custom path set under Windows - please note you must use double slashes under this OS - like so:
"envoy_log_file": "D:\\customEnvoyLogPath\\gm_envoy.log"
Restart Envoy using Launcher. If something doesn't work and logs are not appearing check the writing privileges in the given location.