I don't speak Japanese, but I notice in some manga (like Mr. Fullswing for example), male characters tend to scream "gya" or "guwa" while female characters tend to scream "kya".
Is it true that "gya" sounds masculine and "kya" sounds feminine? If it is, what makes "gya" masculine and "kya" feminine? Is it common practice to use these interjections as such? If a female character uses "gya", does she tend to be describe as boyish (using "ore", "boku" and whatnot)?
Yes, きゃー represents screams of higher tone, and is clearly feminine or childlike. 黄色い声 is usually きゃー. Gay characters often say きゃー in manga, too.
ぎゃー, on the other hand, is not necessarily masculine. When female characters use ぎゃー, it's usually bolder, more urgent, or stronger than きゃー (for example, a dying scream).
In general, voiced consonants tend to be used more often to represent larger, rougher and/or lower-pitched sounds. ガハハ is the sound of vulgar laughter of middle-aged men, whereas キャハハ sounds young/cute/feminine. The same is generally true for onomatopoeia pairs like ドンドン／トントン, ガンガン／カンカン, ギー／キー.
