add images to gallery included in my template

New try, because the title of my last post was a bit misleading…

I have a strange problem…I′m quite new to wordpress, so maybe there is an easy solution to this.

I recently bought this wonderful wordpress theme to use as my new portfolio website. It has a gallery template included. My problem is, that every time I make a new page from the gallery template, and add some images with the media upload feature, these images end up beeing displayed as a “normal” wordpress gallery. They should be displayed in the layout and style given by the template, I think?

to understand better, you can see one of those galleries here:

https://jhochstoeger.com/?page_id=4

see those four images at the top, and then the gallery template further down?
in the gallery template, there are these lines of code for the gallery itself:

<ul class="containsGalleries">
                        <li>
                            <h1>Category Name</h1>
                            <ul class="containsGallery">
                                <li class="G1"><a href="https://www.placeholder-image.com/image/288x288" rel="gallery-1"><img alt="" src="https://www.placeholder-image.com/image/142x142" title="" /></a></li>
                                ...

                            </ul><!-- .containsGallery -->

Any hint? Do I have to build my own template from the one given by the theme, and include the image links “by hand”?

thanks for any info,
Hans