eyeball
眼 centers on the physical eyeball, the organ of sight. It is the concrete, anatomical eye rather than abstract vision or viewpoint.
眼 combines 目 (eye) with 艮, which likely contributes the sound. The character specifically denotes the eyeball.
The left side 目 is an eye, and the right side 艮 looks like a round object with a pupil-like dot. Together they form the eyeball.
For ガン, imagine a gun pointed at an eyeball: gun -> ガン, and the eye is the target.
eye; eyeball
glasses; eyeglasses; spectacles
ophthalmology
eyeball
(a) look; gaze
one eye
naked eye
eyepatch; eye bandage
binoculars; field glasses
(one's) consideration; (taking) notice; interest; attention
main purpose; chief aim; focus
enlightenment; spiritual awakening; opening one's eyes to the truth
presbyopia; age-related longsightedness; farsightedness
focusing on; paying attention to
before one's eyes
bare eyes (i.e. without glasses, contact lenses, etc.); uncorrected vision; unaided vision
single eye; one eye; single lens
clairvoyance; second sight
both eyes; binocular
sparkle of one's eyes