Filter by Category in front end

<select name="g-cats" id="g-cats" onchange="location = this.value;">
  <option value="">Filter by Category</option>
  <option value=".">ALL</option>
  <option value="?gm14[category__in]=4">MEN</option>
  <option value="?gm14[category__in]=5">WOMEN</option>
</select>

I’m doing something like this now, wondering if I can get all the existing categories in the theme file.
thank you.

<select name="g-cats" id="g-cats" onchange="location = this.value;">
	<option value="">Filter by Category</option>
	<option value=".">ALL</option>
	<?php
	global $gmDB;
	$gmedia_categories = $gmDB->get_terms( 'gmedia_category', [ 'fields' => 'id=>names', 'orderby' => 'name', 'order' => 'ASC' ] );
	foreach ( $gmedia_categories as $cat_id => $gm_cat ) {
		echo '<option value="?gm14[category__in]=' . (int) $cat_id . '">' . esc_html( $gm_cat ) . '</option>';
	}
	?>
</select>

perfect thank you

one more thing, this above code will refresh the page upon selection right?
is it possible to filter categories without refreshing the page, ajax like?
thank you.

There are gallery modules where you can display categories as albums (actually all that modules has ‘album’ word in the module’s name) and load them via ajax. Just select [NOT IN] Categories = “Uncategorized” in a “Build Query” popup. There is no other way to load categories via ajax.