  We characterize the symmetric capacity to within 1.7075 bits/s/Hz for the
two-user Gaussian interference channel with feedback. The result makes use of a
deterministic model to provide insights into the Gaussian channel. We derive a
new outer bound to show that a proposed achievable scheme can achieve the
symmetric capacity to within 1.7075 bits for all channel parameters. From this
result, we show that feedback provides unbounded gain, i.e., the gain becomes
arbitrarily large for certain channel parameters. It is a surprising result
because feedback has been so far known to provide only power gain (bounded
gain) in the context of multiple access channels and broadcast channels.
