Study of GS-1427 in Participants With Moderately to Severely Active Ulcerative Colitis

The goal of this study is to learn if GS-1427 is effective and safe in treating participants with moderate to severe ulcerative colitis. The study will compare participants in different treatment groups treated with GS-1427 with participants treated with placebo (Part 1), and participants treated with GS-1427 or ustekinumab alone with participants treated with GS-1427 in combination with ustekinumab (Part 2). The primary objectives of this study are: Part 1: To assess the efficacy of GS-1427, compared with placebo control, in achieving clinical response at Week 12 Part 2: To assess the efficacy of combination therapy with GS-1427 and ustekinumab, compared with GS-1427 and ustekinumab monotherapies, in achieving clinical response at Week 12