During his medical career, Dr. Castellanos served as Medical Director of The Sleep Center at Jupiter Medical Center, President of the Florida Society of Neurology, Chief of Neurology at Palm Beach Gardens Medical Center, and Co-Director of Jupiter Medical Center’s Stroke Center. He founded and served as Medical Director of Carelink Clinic (volunteer clinic bringing medical care and humanitarian services to the underserved, uninsured people living in northern Palm Beach County) and Founder and Medical Liaison of A.W.A.K.E. Sleep Apnea Support Group, Palm Beach County. Dr. Castellanos attended his first Mindfulness Based Stress Reduction (MBSR) course in 1998. He has been practicing mindfulness ever since. In 2009 he began teaching mindfulness workshops and classes and in 2010 began teaching MBSR and related programs. He is also a personal mindfulness trainer. He is currently teaching at a variety of facilities throughout South Florida, including the Sari Center for Integrative Cancer Care, U of Miami Osher Center for Integrative Medicine, UMindfulness, Mindful Kids Miami, FIU College of Medicine, Nova South Eastern University, U of Miami Miller School of Medicine, Florida Dept of Health, Children's Council of Palm Beach, College of Advanced Judicial Studies and Connected Warriors, a non-profit empowering Servicemembers, Veterans, and their Families. Gus is the mindfulness teacher for GEN2050, a program for under-resourced teens inspiring Entrepreneurship, Leadership, Environmental Stewardship, STEM, and Social Responsibility. Over the past ten + years, Gus has facilitated programs for FAU School of Social Work, Palm Beach County Fire Rescue, Miami Dade Fire Rescue, Town of Jupiter, US Army, United Way of Broward, Trustbridge Hospice of Palm Beach County, Jupiter Medical Center, College of Advanced Judicial Studies, Akerman, LLP, Searcy, Denny, Scarola, Barnhart Law Firm, Conference of County Court Judges of Florida, Hanley Treatment Center, A New Day Treatment Center, The Wellness Center at Ultima Fitness Center, American Express, SocialCode (Code3), Berkowitz Pollack Brant Advisors and Accountants, Tiger 21, Junior League of the Palm Beaches, NCH Healthcare in Naples (a Mayo Clinic affiliate), Harbour's Edge - A Lifespace Community, Department of Children and Families, St Andrew's School in Boca Raton, Christopher Columbus High School in Miami, Florida Atlantic University School of Social Work, eClerx LLC, Substance Abuse Prevention Coalition of Monroe County, Florida Dept of Health nurses, and The Mindful Path. He designed a Mindfulness-Based program for a local prison in 2011 and continues to provide twice-monthly mindfulness sessions there. He lectures widely and writes a bi-monthly South Florida Mindfulness newsletter. He founded a local mindfulness teacher sangha in 2011 and was one of the meditation teacher trainers at Innergy Meditation in South Beach in 2019. Gus co-organized and co-hosted the "A Mindful South Florida" event in December 2109. This event brought together nationally known leaders in the field of mindfulness and mental health, trauma, school violence, and criminal justice. Over 600 people registered for the main symposium and 7 workshops throughout Miami-Dade, Broward, and Palm Beach Counties. Gus is involved in mindfulness research with the University of Miami Miller School of Medicine, FIU College of Medicine, and Nova South Eastern University and in the past with Amishi Jha's Mindfulness Based Attention Training program at the University of Miami Neuroscience Lab. He also serves on Palm Healthcare Foundation, Inc.’s "Train the Brain" campaign Advisory Board and planning committee.