var fs = require('fs'),
emailsFile = './mails.txt';
fs.readFile(emailsFile, function (err, data) {
if (err) throw err;
let emails = data.toString(),
gmails = emails.match(/(\w)+
@gmail\.com/g),
gmailList = gmails.join('\n');
fs.writeFileSync('gmails.txt', gmailList, "utf8");
});