Learn Icelandic: Atvinnuviðtalið gekk mjög vel. - The job interview went very well.

Yes, the literal meaning of the infinitive verb ganga is "to walk". However, Icelandic uses it idiomatically in the exact same way English uses "to go" when talking about how an event or situation progressed. Therefore, gekk mjög vel translates naturally to "went very well".

How do we know atvinnuviðtalið is a neuter noun that takes the -ið ending?

In Icelandic compound words, the grammatical gender is always determined by the last noun in the combination. This word is made of atvinna (feminine, meaning "employment") and viðtal (neuter, meaning "interview"). Because viðtal is neuter, the entire compound noun becomes neuter and takes the neuter definite article -ið.

Why is there a -u- in the middle of atvinnuviðtalið?

This is the genitive (possessive) connecting vowel. The feminine noun atvinna becomes atvinnu in the genitive case. When linking nouns to make a compound word in Icelandic, the first word is often put into the genitive case, so this literally translates to "interview of employment".

Could I say gekk mjög gott instead of gekk mjög vel?