Inclusion Criteria:
* Between the ages of 19 and 85
* Has stage IV or unresectable GI malignancy
* Resides within a 170-mile radius of Omaha, NE
* Speaks English
* Eastern Cooperative Oncology Group (ECOG) performance status of 0-3
* Life expectancy ≥ 8 weeks as determined by referring oncologist
* Ability to provide written informed consent and comply with study procedures
* Awareness of the neoplastic and likely incurable nature of his/her disease
* Has one family member willing to participate in measures
* Agreeable to using an adequate method of contraception or birth control from the time of enrollment to 24 hours following the psilocybin session (male and female participant of childbearing potential, defined as age \<55 and menses within the prior 2 years with intact ovaries and uterus)
* Negative pregnancy test result (female participants)
Exclusion Criteria:
* Long-term or unstable psychiatric illness that would prevent safe cessation of psychotropic drugs including MAOIs, lithium, or anti-psychotics
* Severe symptoms of depression or anxiety warranting immediate impatient evaluation or treatment
* High-risk of suicide, as measured by Columbia Suicide Severity rating Scale
* Current or prior history of schizophrenia, psychotic disorder (unless substance induced or due to medical condition) or bipolar I or II disorder
* First-degree family member with schizophrenia, psychotic disorder (unless substance induced or due to medical condition) or bipolar I or II disorder
* Conditions known to be incompatible with establishment of rapport or safe exposure to psilocybin including dissociative disorder, borderline personality, traumatic brain injury, obsessive compulsive disorder, anorexia nervosa, or bulimia nervosa
* Alcohol or recreational drug abuse disorder, excluding caffeine and nicotine
* Known CNS metastases or other major CNS disease such as seizure disorder, dementia, Parkinson's disease, multiple sclerosis
* Receive treatment in another clinical trial involving an investigational product for the treatment of cancer during the interventional stage of the protocol
* Advanced hepatic dysfunction as indicated by a Child-Pugh Score of C
* Renal dysfunction as indicated by creatinine clearance \<40 ml/min using the Cockroft-Gault equation
* Cardiac or circulatory dysfunction defined as uncontrolled hypertension (systolic blood pressure \> 140 or diastolic blood pressure \>90 mmHg on three separate readings), angina, stroke or myocardial infarction in the prior 6 months, or claudication
* History of seizures
* Unable to skip a meal (lunch), or have diabetes which requires administration of medication more than twice daily, or with symptomatic hypoglycemia within the prior 30 days
* Pregnant or breastfeeding
* Currently using any of the following potent metabolic inducers or inhibitors
* Inducers: rifampin, rifabutin, rifapentine, carbamazepine, phenytoin, phenobarbital, nevirapine, efavirenz, St. Johns Wort, or Paclitaxel and dexamethasone (latter two permitted if 5 half-lives have passed between last dose and psilocybin session)
* Inhibitors: All HIV protease inhibitors, itraconazole, ketoconazole, erythromycin, clarithromycin, troleandomycin
* Metal in body (i.e. hearing aid, cardiac pacemaker, bone plates, braces, non-removeable piercings/implants, etc.) claustrophobia, inability to lay still for one-hour, or any other condition that would preclude MRI scanning
