A quaker is an unripe or immature coffee bean. It has a wrinkled surface and is smaller and less dense than a ripe bean.
When coffee beans are roasted they darken in colour; dark coffee like espresso has been roasted longer than lighter blends. Quakers, however, remain pale and yellowish no matter how long they are roasted.
Quakers taste bad, and ruin the flavour of otherwise good coffee, so roasted beans are rated for, among other things, how many quakers are found in a batch. Fewer quakers is obviously better, and coffee which contains few quakers commands a higher price than quaker-filled batches. Top quality expensive coffee will contain no more than a few quakers per pound; low quality cheap coffee can contain forty or more. If you're curious about your favourite coffee, pour the beans out on the counter and have a look; it's easy for even the untrained eye to pick out the pale, immature beans.
So the reason why that gas station coffee tastes so bad is that it's made with cheap coffee that contains a high proportion of quakers.