I saw it in the context of ゲヒゲヒ笑う and would like to know what kind of laugh it is. Some quick Googling seems to indicate it might be like laughing uncontrollably maybe but I can't find any sources.
Though 「ゲヒゲヒ」 is not such a common onomatopoeia, it would describe a loud and coarse kind of laugh mostly (but not always) produced by men of the same kind.
We already have 「ゲラゲラ」 and 「ヒヒヒ」for laughing sounds which are far more common and this fact enables one to guess what 「ゲヒゲヒ」 would mean even if one had never heard/seen it (just as @Chocolate's comment above would suggest).
「ゲヒゲヒ」 would definitely not be a quiet, well-mannered, refined or classy kind of laugh. These 'gentler' qualities are rarely, if ever, expressed by onomatopoeias containing a ガ, ギ, グ, ゲ or ゴ.
IMHO, this topic is somewhat related to this Q&A in terms of how certain sounds feel to the native speakers.