Do you mean global available asset metadata? Then:
Copy the file cp-assets.tag (source) into /config/tags and modfiy it.
If you copy and paste some sample code:
you can add your custom fields.
<div class="uk-form-row">
<label class="uk-text-small uk-text-bold">{ App.i18n.get('Orientation') }</label>
<input class="uk-width-1-1" type="text" bind="asset.orientation">
</div>
Elseif you want to add a more meta data to the gallery field, you can add more meta fields in the field options:
{
"meta": {
"title": {
"type": "text",
"label": "Title"
},
"title": {
"orientation": "text",
"label": "Orientation"
}
}
}