If you want to make your own Roman style garum, here you go:
I'm sure oriental fish sauce comes close enough. They are all over the internet from Thailand, Vietnam, Malaysia, Japan and China. As far as I can see, they are all made from various fermented fish in salt, with sundry herb flavourings. Now all I need to know is how and when to use them.
It appears to me (but I am not an English native speaker) that the officially correct translation refers to "a sauce made of salty fish", with the adjective "salty" apparently referred to the fish, while the concept of "Garum salsum gusto" is that what I taste is a sauce made of fish and that such a sauce happens to be salty (but could also be tasteless). Therefore, shouldn't be more correct to translate "Garum salsum gusto" as "I taste the fish salty sauce", with the adjective "salty" refferred to the sauce?
"I taste the salty fish sauce." can mean either that the fish sauce is salty or that the sauce is made of salty fish. You can't separate two connected nouns with the meaning of "of" with an adjective. You can use a participle to separate it though. Look at "a spicy tomato ketchup". You can't say "a tomato spicy ketchup" but you can say "a tomato based spicy ketchup".