Add this to $SNAP/bin/gpt.options ($SNAP is the directory where snap is installed)
-Dhttp.proxyHost=http.proxy.ip
-Dhttp.proxyPort=NNNN
-Dhttps.proxyHost=https.proxy.ip
-Dhttps.proxyPort=MMMM
where http(s).proxy.ip is the IP address of your proxy server and NNNN, MMMM their respective ports.
Check you system set up to discover proxy settings, i.e. via:
echo $http_proxy