A LOTR character: Gaffer Gamgee
A Hobbit character: Gollum
A place in Middle-Earth: Gondor
Things from LOTR: Grond
Beasts/Monsters: Glaurung the Golden
A Silmarillion character: Gwindor
Another Barrow Downer:
Gwathagor
Actor from LOTR movies: how about... Gary Stu?
A weapon: Glamdring
A dwarven character: Gloin
An elf: Gildor
A hobbit: Gorbadoc Brandybuck
A human: Grima Wormtongue
An elf-human hybrid: Elwin
g ...well, it works from the other end...
A maia: Gandalf
A place in Aman (including Tol Eressea): Gardens of Lorien
A battle: The Glorious Battle (is that the English for Dagor Aglareb?)