Website full-Width size Gmedia Pro

If your theme have full width templates you can create Page with a gallery and choose that template. Check maybe there is some option in your theme to at least disable empty sidebar on your Posts, so the content block will be wider.

If you can’t do things above then try add code below to the Custom CSS field of the gallery:

#top #wrap_all #main .sidebar {
    display: none;
}

#main .container .av-content-small.units {
    width: 100%;
}

#main .content .entry-content-wrapper {
    padding-right: 0;
}

If you need full window gallery from side to side then add also this code:

#main .template-blog .post .entry-content-wrapper {
    overflow: visible;
}

#main .entry-content {
    width: calc(100vw - 40px);
    margin-left: 50%;
    transform: translateX(-50%);
}