<template>
...
<gmap-custom-marker
    :id="'mark_' + index"
    :title="'Marker Title'"
    :marker="get_clean_lat_lng (0,0)"
    :clickable="true"
    class="g_marker"
    alignment="center"
    :z-index="999"
  >
    <map-pointer id="'point_' + index" :icon="'arrow'" :color="'red'" :tool-text="''" />
</gmap-custom-marker>
...
</template>


<script>
...
mounted() {
    this.$refs.baseMap.$mapPromise.then(map => {
        console.log(document.getElementById('mark_' + index)) // null
    })
  },
...