function doGet(e){
var fileName = e.parameter.file;
var today = Utilities.formatDate(new Date(), "JST", "yyyyMM");
var url = "https://280blocker.net/files/"; + fileName.replace("yyyyMM", today);
var res = UrlFetchApp.fetch(url);
var out = ContentService.createTextOutput(res);
return out;
}

280blocker_domain_ag_yyyyMM.txt

どうだろ?何処がおかしい?