This study is designed to examine the efficacy of erlotinib versus gemcitabine/cisplatin as a second-line treatment in NSCLC patients from the ENSURE trial (NCT01342965). Patients previously treated with gemcitabine/cisplatin will be given erlotinib daily until disease progression or unacceptable toxicity occurs. Patients previously treated with erlotinib will be given cisplatin on Day 1 and gemcitabine on Days 1 and 8 of 3-week chemotherapy cycles until disease progression, unacceptable toxicity or up to 4 cycles (whichever comes first).