The u is superfluous in Zamasu, it's the same with a ton of words when you translate from Japanese to English. Think of Beerus/Birusu or Cell/Seru. Both are technically correct, but a lot of words just have the u removed when they get localized.
So Zamasu/Zamas and Gowasu/Gowas are both valid. I'd bet Funi will go with Gowas/Zamas when they're pronounced in the dub, though.