On Sat, 2 Feb 2013 07:38:36 -0800 (PST), Robert Macy
wrote:

Over time something has changed here. The closed door to the garage
now lets light in around it.

A couple of years ago, if I left the garage light on at night - could
not tell, but now, there's a halo around the door. From examing the
gasketing, it appears to still be ok, not appreciably shrunk Hwoever,
simply pulling on the door handle to 'more close' door blacks out the
garage light so the halo disappears.

Examining the metal part mounted in the door frame, it does not appear
to be something that can easily be slightly moved in, to 'tighte' the
door.

What's a good way to fix this problem?

Shim the strike plate, or shim the back of the ?? metal part onn the
door itself? Or, adjust some mounting somewhere?

The strike plate has a small tab, that has a small slot in it. Use a
small screwdriver and bend the tab closer to the latch bolt. This
will tighten the door a bit and may be all you need to close the gap.

I would start there.