Retrieve all slot data from performance.getEntries. Fire it once all the slots have been requested. If you page happens to have a LOT of network traffic. you may want to expand the number of entries allowed using the following script before the first GPT request. window.performance.setResourceTimingBufferSize(10000);

getSlotsByRequestData

/**
 * We collect this data from the performance entries instead of directly from GPT because the targeting
 * can can change between requests.  Pulling data from googletag after a refresh will only give you the
 * cuurent state of GPT.
 */

(() => {
  function getUrlParams(search) {
    let hashes = search.split('&').filter(item => {
      return /^enc_prev_ius|^iu_parts|^prev_iu_szs|^prev_scp|^cust_params|^correlator/.test(item);
    });
    let params = {};
    hashes.map(hash => {
      let [key, val] = hash.split('=');
      return params[key] = decodeURIComponent(val);
    });
    return params;
  }

  function arrayToObject(obj, entry) {
    obj[entry[0]] = entry[1];
    return obj;
  }

  function getPageTargeting(entry) {
    return ((entry.cust_params && entry.cust_params.split('&')) || [])
      .map(entry => entry.split('='))
      .reduce(arrayToObject, {});
  }

  function createSlots(entry) {
    const slots = [];
    entry.prev_iu_szs.split(',').forEach(slot => {
      slots.push({
        sizes: slot.split('|')
      });
    });
    return Object.assign(entry, {
      slots: slots
    });
  }

  function getSlotData(entry) {
    const slots = [];
    const pageTargeting = getPageTargeting(entry);
    (entry.prev_scp.split('|') || []).forEach(slot => {
      slot = (slot.split('&') || []).map(el => el.split('=')).reduce(arrayToObject, {});
      slots.push(slot);
    });
    entry.slots.forEach((slot, i) => {
      const parts = entry.iu_parts.split(',')
      const ius = entry.enc_prev_ius
        .split(',')
        .map(iu => iu.split('/'))
        .map(iu => iu.map(part => parts[part]).join('/'));

      slot.targeting = slots[i];
      Object.assign(slot.targeting, pageTargeting);
      slot.adUnit = ius[i];
      slot.correlator = entry.correlator;
    });
    return {
      slots: entry.slots,
    };
  }

  function combineEntries(prev, curr) {
    return prev.concat(curr.slots);
  }

  function getGptData() {
    const regex = new RegExp('pubads.g.doubleclick.net/gampad/ads');
    return window.performance
      .getEntries()
      .filter(entry => regex.test(entry.name))
      .map(entry => entry.name.split('?')[1])
      .map(getUrlParams)
      .map(createSlots)
      .map(getSlotData)
      .reduce(combineEntries, []);
  }

  console.log('GptData', getGptData());
})();