Casual katakana noun for a man or fellow. As a standalone word it can sound English-flavored or translation-like; it is especially easy to encounter in expressions such as ナイスガイ and タフガイ.
He is friendly and is called a nice guy by everyone.
The protagonist of that movie is a typical tough guy.
ガイド means "guide" (a person or booklet that leads/explains), while ガイ means "guy"; the two katakana words look and sound similar but are unrelated in meaning.
nice guy
No language of origin is specified. The katakana spelling corresponds in use to English-style “guy,” but no specific borrowing history is provided here.