[Skylands]

Gary - Possibly taken from Garry Oak, a type of oak tree. Also possibly derived from Shigeru.
Giovanni - Italian Giovanni, equivalent to John ("God is gracious"); gio sounds like geo, from Greek γῆ gē, earth. May also be a play on his resemblance to a Mafia boss, who are typically Italian.
Blaine - Sounds similar to blaze or flame.
Erika - From the Erica genus.
Surge - Surge as in electrical surge. Also, "Sarge" (as in sergeant, another military rank) seems to apply. 'Lt.' may also refer to lightning as the game describes him as "The Lightning American".
Lorelei - The name of a siren whose beauty and voice lured sailors of the Rhine River to their deaths. In the anime, her name is Prima.
Bruno - Means "brown haired". It's similar to brawn, bruiser and to brunt.
Chuck - To throw, or possibly from nanchaku. Possibly a reference to martial artist Chuck Norris.
Jasmine - A jasmine is a sweet-smelling flower; the mine refers to mineral, or an actual mine.
Whitney - Similar to the word white.
Koga - From 甲賀流 Kōga-ryū, a school of ninjutsu.
Will -From "willpower", mental strength.
Lance - Possibly from a lance, used by knights, who fought dragons in legends and fairy tales.
Karen - From 花梨, a Burmese Rosewood
Agatha - Ironically means good and kind, but likely chosen for similarity with aghast, ghost or hag, or possibly a reference to Agatha Christie, a popular British crime writer, a possible reference to the Ghost or even the Poison typing of her team.

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I know some of those still don't exactly make sense with the type of Pokemon each Gym Leader uses, but close enough.