I guess there is room for more global input fields "before" and "after" content for all the X3 modules (including gallery). For now, you would have to add it with javascript:
Paste this into Settings > Footer. Feel free to adjust the form:
<form data-abide id="clone-form" class="contactform clone-form" style="display: none">
<div>
<input type="text" name="name" placeholder="Name" required>
</div>
<div>
<input type="email" name="email" placeholder="Email" required>
</div>
<div>
<textarea rows="6" name="message" placeholder="Message" required></textarea>
</div>
<button type="submit">Send</button>
</form>
Paste this in Custom > Javascript. It clones the form and places it under the gallery for all pages that have a gallery:
function x3_load_page(){
var gallery = document.getElementsByClassName('gallery')[0];
if(gallery){
var form = document.getElementById('clone-form').cloneNode(true);
form.style.display = 'block';
gallery.parentNode.appendChild(form);
}
}
Add some CSS to style the cloned form, making sure it's not too wide, has space and is centered. You can customize of course:
.clone-form {
max-width: 500px;
margin: 0 auto 60px;
}
This solution is not tested for all circumstances. It will display under ALL galleries, including if you are using slideshow intro or have the gallery places above context. This is after all how it would work if there was a setting to add content before/after the gallery module.