What are the real symptoms of GI cancer?

GI too broad to say. Gi goes from swallowing tube to anal aperture, and a bunch of juice squeezers along the way(pancreas, bile ducts and gall bladder). Blockage of the tubes cause nausea, vomiting, pain. Many ooze blood, but so do benign things. Colorectal blocks cause constipation. Vomiting and pooping blood should not be put off, but not always caused by cancer.
Which GI cancer? There are a lot of GI cancer. The symptoms will depend on the type of cancer, the location, the stage etc. Pain either from obstruction/ narrowing of the GI lumen, problems with digestion ( swallowing problem- if located at the esophagus) , weight loss, bleeding can occur. If you can be more specific you will get a better answer.