make sure your local server is configured for port 8080 http requests (for example)
then on your watch, when you set the server choose Custom, and address of the server http://your_localserver:port
Also, make sure you are following this:
https://blynkapi.docs.apiary.io/#reference/0/write-pin-value-via-get/write-pin-value-via-get
if you are writing this in your browser
http://your_localserver:your_port/yourauth_token/update/V0?value=1 (or zero or whatever)
where obviously you replace the BOLD characters with your values, your device SHOULD act. if not, look again into your server config file. It has nothing to do with the GearBlynk app or the watch.