I am not sure if I have used the g.button correctly. Please let me know if this is the issue:
###Gallery buttons
#g.button("cg_1")
#g.button("cg_2")
g.button("cg_3")
g.button("cg_4")
g.button("stretcher_cg")
g.button("fire_backstory_cg")
## Declarations for the images used in the gallery. May or may not
## be needed if you're using Ren'Py's automatic image names.
image stretcher_cg = "stretcher_cg.png"
image fire_backstory_cg = "fire_backstory_cg.png"
image cg_3 = "gallery_test.png"
## This is just the button name + _thumb
image stretcher_cg_thumb = AlphaMask(Transform("stretcher_cg", xysize=gallery_thumb_size), "gui/gallery/mask.png")
image fire_backstory_cg_thumb = AlphaMask(Transform("fire_backstory_cg", xysize=gallery_thumb_size), "gui/gallery/mask.png")
image cg_3_thumb = AlphaMask(Transform("cg_3", xysize=gallery_thumb_size), "gui/gallery/mask.png")
screen gal_scenes_1:
###make only 4 buttons
add g.make_button('stretcher_cg','stretcher_cg_thumb')
add g.make_button('fire_backstory_cg','fire_backstory_cg_thumb')
add g.make_button('cg_3','cg_3_thumb')
add g.make_button('cg_4','cg_4_thumb')