這個拼法是2個不同的字· 2 words spelled “gaffer”
One way to picture it: the gaffer is the person with the tool in their hands at the sharp end of the work, and the one whose decisions the others follow. The supplied evidence reads the word as gaff (a hooked pole) + -er. On early film sets the light was changed by opening and closing the gaff cloths, and the person who ran that became the gaffer. At the glass bench, the worker holding the tools and shaping the hot piece carries the same name.
The evidence packet derives this gaffer from gaff (a hook) plus -er, and notes that natural light on early film sets was adjusted by opening and closing gaff cloths or gaff flaps; the glass sense is filed under the same root without further explanation.
手上的資料把這個 gaffer 拆成 gaff（鉤子、鉤桿）加 -er，並提到早期片場是靠開合 gaff cloths 來調光。玻璃師傅的意思被歸在同一條字源下，但來源沒有說明過程，所以那一段只能當成待查。
Wiktionary, "gaffer" (etymology 1)
One branch
現場動手、而且說了算的人
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head of lighting (film, TV) · noun
The person in charge of the lighting on a film or television set. The gaffer leads the lighting crew, decides which lamps go where, and makes the picture look the way the camera team wants it to look.
燈光師
One way to picture it: gaffer started as something you call a man, not a job you give him. The supplied evidence reads it as a godfather, with the pulled toward grandfather. So the picture is close to 'the old fellow' — the man in the village everyone knows, or the most senior hand on the work team. Said out loud it is warm, a bit teasing, and never formal. Follow the age side and you get 'old man'; follow the seniority side and you get 'the boss'.
The evidence packet says this gaffer is likely a contraction of godfather, with vowels influenced by grandfather, comparing French compère and German Gevatter. The packet hedges with 'likely', and so do we.
手上的資料說，這個 gaffer 可能是 godfather 的縮短，母音受到 grandfather 影響，並拿法文 compère、德文 Gevatter 來比對。來源用的字是「likely」，所以這裡也只當成可能的路線。
Wiktionary, "gaffer" (etymology 2)
One branch
喊一聲「那位老的」
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old man · noun
An old man. This is British English and informal, and the sources mark it as humorous — it is the kind of word used in a joking, friendly way, often in old stories or about an old man in a village.
老頭子（戲稱）