IGE stands for Independent Government Estimate. The FAR/DFARS doesn't provide a definition but DFARS 236.204 does discuss Government Estimates. DFARS 236.203 -- Government estimate of construction costs states: "Follow the procedures at PGI 236.203 for handling the Government estimate of construction costs."
PGI 236.203 -- Government estimate of construction costs states: (1) Designate the Government estimate as “For Official Use Only,” unless the information is classified. If it is, handle the estimate in accordance with security regulations.
(2) For sealed bid acquisitions—
(i) File a sealed copy of the Government estimate with the bids. (In the case of two-step acquisitions, this is done in the second step.)
(ii) After the bids are read and recorded, remove the “For Official Use Only” designation and read and record the estimate as if it were a bid, in the same detail as the bids.
Also, see FAR 36.204 and DFARS 236.204 regarding "Disclosure of the magnitude of construction projects."