Your comparison is drastically wrong. GM's profit in 1955 was a similar percentage vs US GDP, as what Apple is today.

US GDP in 1955 was around $400 billion (it didn't reach $3t until the 1980s). GM's profit in 1955 exceeded $1 billion.[1] That produces a ratio very similar to Apple's today. That's despite immense global growth since 1955 giving Apple an enormous global market to play in. Most of GM's profit was derived from the US domestic market. If we went just by Apple's US profit today, their ratio would be slashed to closer to 1/1000.

US GDP in 2018 will be roughly $20.5 trillion. Apple's ratio against that is about 1/360 (their profit will be closer to $57b-$60b for 2018 than $50b).

[1] http://content.time.com/time/magazine/article/0,9171,807967,...

GM was selling cars mostly in the US, while more than 60% of Apple's sales happen outside the US. There's still quite difference, but a little smaller than the raw numbers.