You don't have to be sent to gitmo to be declared an "enemy combatant." It's this designation that loses your Habeas Corpus.
Jose Padilla, a US citizen in Chicago, was arrested in 2002 for "aiding terrorists." He was held without charges for 4 years. His petition for Habeas Corpus was denied by the court because he was designated by Bush as an "illegal enemy combatant." He was told to challenge that designation first in order to petition for Habeas Corpus.