I think the prevalence of "Garand" comes from the same place as Deagle and PP7
The vidja games.
Im pretty sure it was referred to as the Garand in the old medal of honors and call of duty's.
For what its worth i call it the Garand, and i also say .45 Long Colt. Not because of video games, but because thats the terms my dad used when first describing these things to me at a young age.
The question is, where did he pick it up to pass it on?