Скорее всего, вам нужно сделать вот это
const data = JSON.parse(response.getContentText());
function myFunction() {
const response = UrlFetchApp.fetch("https://suppliers-api.wildberries.ru/api/v2/stocks...", options);
const data = JSON.parse(response.getContentText());
console.log(data);
/**
{
"stocks":[
{
"subject":"Брюки",
"brand":"R.O.S.E.",
"name":"Брюки",
"size":"42",
"barcode":"2036686215443",
"article":"292022",
"warehouseName":"Брест",
"barcodes":[
"2036686215443"
],
"stock":0,
"warehouseId":198679,
"id":145904543,
"chrtId":145904543,
"nmId":90612284
}
],
"total":1
}
*/
console.log(data.stocks[0]);
/**
{
"subject":"Брюки",
"brand":"R.O.S.E.",
"name":"Брюки",
"size":"42",
"barcode":"2036686215443",
"article":"292022",
"warehouseName":"Брест",
"barcodes":[
"2036686215443"
],
"stock":0,
"warehouseId":198679,
"id":145904543,
"chrtId":145904543,
"nmId":90612284
}
*/
}