CSS snippet to control the spacing/ padding of the Masonry grid (set to “large)

choose the large gap and place that css into your child-theme quick css:
(masonry-gallery – or do you mean the masonry-entries?)

#top .av-large-gap.av-fixed-size .av-masonry-entry .av-inner-masonry {
    right: 50px;
    bottom: 50px;
}

i try to find a solution for av-flex-size now…

#top .av-large-gap.av-flex-size .av-masonry-entry .av-inner-masonry {
  position: relative;
  margin-right: 20px !important;
  margin-bottom: 20px !important;
}

set up for mobile devices :

.responsive.avia_mobile #top .av-large-gap.av-fixed-size .av-masonry-entry .av-inner-masonry {
    right: 20px;
    bottom: 20px;
}

or if you mean for small screens – then a media-query is the means of choice instead:
f.e.:

@media screen and (max-width: 767px) {
  #top .av-large-gap.av-fixed-size .av-masonry-entry .av-inner-masonry {
      right: 20px;
      bottom: 20px;
  }
}