#1. Use this code to Code Injection > Footer (or Page Header Code Injection)

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.7.0/jquery.min.js"></script>
<script>
$(document).ready(function(){
  $(".image-slide-title").each(function(){
    $(this).html($(this).text());
  });
});
</script>
<style>
  .image-slide-title a {
    border-bottom: 1px solid;
}
</style>

#2. Next, edit Gallery Block > Edit Image Title > Use this format

Just some text <a href="https://google.com">button</a>

#3. Result