This study will primarily evaluate the safety and efficacy of GS-5745 in adults with active Crohn's disease. The study will consist of a Blinded Treatment Period of 8 weeks followed by an Open Label Extension. Participants who complete the Blinded Treatment Period will be eligible to enroll in the optional Open Label Extension for an additional 44 weeks. Participants who complete Week 52 assessments will be eligible to enter the Extended Treatment Phase to continue treatment with GS-5745 for an additional 156 weeks.