Did it stumble, then quit or just shut off immediately??
Did you get any other strange indications on the dash - battery, ABS, or other light?
What condition is the battery? New? Alternator working? What's the battery voltage at an idle - around 13.8vdc? What's the battery voltage with the car off?

A fuel pump problem won't affect the gas gauge. This sounds electrical - low battery due to bad battery or alternator.

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FrankenBenz: '87 560SEC with an '02 cammed LS6 and custom 4L60E.
Bling: '87 560SEC, ported/polished, lowered
Lisa: '87 560SEL, lowered