I need one that
a. can show thumbnails
b. can expand on click or hover (lightbox style)
c. shows caption
d. works in mobile


Very Simple Responsive Image Gallery (HTML CSS) seems to have everything, though no auto-advance



https://www.geeksforgeeks.org/css-image-gallery/



Creating an interactive image gallery with html and css
uses javascript snippet to obtain full screen images on click.  

30+ CSS Image Gallery Examples Code Snippet various possibilities.

#3 is a bit gimmicky but could have certain uses.
#27  though you have to click to close, so no automatic advance.
#28 (CSS and js)  looks good, but no demo
#34 ok but a bit weird
#35 but I think the images remain fairly small.