Как я могу возвратить значение от GM_xmlhttprequest?

.block {
  border: 1px solid red;
  display: flex;
  flex-wrap: wrap;
  max-width: 900px;
  justify-content: flex-start;

}

.block__item {
  background: grey;
  height: 20px;
  border-bottom: 2px solid white;
  border-right: 2px solid white;
  flex-grow: 1;
  flex-shrink: 0;
}

.block__item:last-of-type {
  border-right: none; 
}

<div class="block">
  <div class="block__item"></div>
  <div class="block__item"></div>
  <div class="block__item"></div>
  <div class="block__item"></div>
  <div class="block__item"></div>
  <div class="block__item"></div>
</div>
<div class="block">
  <div class="block__item"></div>
  <div class="block__item"></div>
</div>
<div class="block">
  <div class="block__item"></div>
</div>