I’ve got the same problem, somehow there’s an extra space of 6 px at the bottom of each item. I can’t seem to figure out where it comes from and before I edited the Gallery today it worked fine with even gutters. Thank you for suggesting the custom css (insert margin-bottom with 6px less space than the gutter width), that worked ??
update: I’ve tried to solve the problem and it seems to be related to line-height. The following custom css removed the extra (6) pixels for me:
#foogallery-gallery-[insertgallerynumberhere] div.item {
line-height:0;
}