cover, lid, flap
蓋 centers on something that closes over an opening or object: a cover, lid, or flap that conceals or protects what is underneath.
蓋 originally combined 艹 (grass) with 盍, which likely contributed the sound and the idea of covering. The character expressed a cover made of grass or a lid.
The grass crown 艹 sits over 盍, which looks like a container with a lid (皿 with something on top). Picture a woven grass cover placed over a dish: 蓋 is a lid or flap.
For ガイ, imagine a guy lifting a heavy lid: guy -> ガイ, and he covers the pot with a big flap.
cover; lid; cap
skull; cranium
cranium; skull
scab; crust
palate
canopy; dome
probability; likelihood
certainly; really; truly; indeed
apron of a gun; cover for the touch-hole on a cannon or matchlock
kneecap; patella
epiglottis
slip-on lid; lid going over the edges
lid (of a box, container, etc.); cover; cap
trap door; removable floor boards
ascending palatine artery
descending palatine artery
hunting-case watch
hunting-case watch
inner lid
Veronicastrum sibiricum (species of flowering plant)