Oxford Handbook of Gastroenterology and Hepatology by 
Greenberger's CURRENT Diagnosis & Treatment Gastroenterology, Hepatology, & Endoscopy, Fourth Edition by 
Crash Course Gastrointestinal System Updated Print + EBook Edition by BMJ Best Practice has a dedicated topic for Gastroenterology and Hepatology with subjects organised either by alphabetical order or emergency. An OpenAthens account is required to access this resource.
Gastroenterology and Hepatology | BMJ Best Practice
NICE (National Institute for Health and Care Excellence)
In the NICE you can find various guidance and advice on gastrointestinal (see below the link to them)
gastrointestinal | Search results | NICE
BNF (British National Formulary)
Accessible online, in the BNF you can find information by drugs and you can find treatment summaries.
BNF (British National Formulary) | NICE
