Goer is go with -er on the end: the one that goes. Hold that one picture — a door standing open with something moving through it — and the rest is only a question of what kind of going you mean. Going to the same place again and again gives you cinema-goer. Going strongly and gives you a car that people call a good little goer. An old and rude piece of British slang took the same idea and pointed it at a woman's private life.
From the word’s history
Wiktionary records goer as Middle English goere, simply go plus the -er ending that makes a doer out of a verb, and compares German Geher (goer, walker). So the plain agent noun really is the oldest layer here.
Wiktionary 記載 goer 來自中古英語 goere，就是 go 加上 -er 這個「做某事的人」字尾，並拿德語 Geher（走路的人）來比較。所以「會去的那一個」確實是最底下的一層，其他用法都疊在它上面。
Wiktionary (kaikki extract, source_ref kaikki:3f6f69a190036ae4:617454752): 'From Middle English goere, equivalent to go + -er. Compare German Geher.'
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