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

Key Inclusion Criteria: * Individuals have Ulcerative Colitis (UC) with symptoms of at least 90 days duration before randomization, with the diagnosis confirmed by endoscopy and histology at any time prior to randomization. Documentation of endoscopy and histology consistent with the diagnosis of UC must be available in the source documents. * Individuals have UC with minimum disease extent of 15 cm from the anal verge. * Individuals have moderately to severely active UC as determined by endoscopy occurring during screening with a total modified Mayo Clinic Score (mMCS) of 5 to 9 points, including a centrally read endoscopic subscore of at least 2. * Individuals have an inadequate response or loss of response or are intolerant to at least 1 of the following UC treatments: corticosteroids, immunomodulators, or advanced therapy. * Individuals have an inadequate response or loss of response or are intolerant to \< 3 AT mechanisms of action for UC (use of 2 or more AT with the same mechanism of action, eg, 2 TNF-α inhibitors, counts as 1 mechanism of action) Key Exclusion Criteria: * Have a current diagnosis of Crohn's Disease (CD) or clinical findings suggestive of CD, diagnosis of indeterminate colitis due to etiologies such as an enteric pathogen, or lymphocytic or collagenous colitis. * Have a current diagnosis of toxic megacolon, symptomatic colonic stricture, acute severe colitis, fulminant colitis, or abdominal abscess at screening or randomization. * Have any history of exposure to vedolizumab or other integrin antagonists * Requirement for ongoing therapy with or use of any prohibited medication as specified in the protocol Note: Other protocol defined Inclusion/Exclusion criteria may apply