What does the condition "f=100Hz" mean for the Gain Bandwidth Product?
It means that they test the gain at 100Hz, assume that it's slope is 20dB/decade at that point, and extrapolate the GBW product from there.

At 150Hz, the amplifier gain (and the loop gain in a buffer) is only a factor of 13 or so.  That means that any inherent distortion in the amplifier is only divided by a factor of 13 -- and the whole notion of an op-amp is "overcome problems deriving from a crappy amplifier with tons of gain".  "Tons of gain" varies by what you're trying to do, but I would say that \$2^n\$, where \$n\$ is the number of bits of precision you're interested in, is a good first approximation.  So, if you're feeding a 16-bit ADC, you'd expect good performance with a loop gain better than 65000.