I'm really concerned because every morning I wake up coughing up this thick, sticky phlegm that's jelly-like. It varies in color, sometimes clear, other times white or grey. What could be causing this, and should I be worried?

This type of mucoid phlegm is typical of allergic bronchitis. Since you have gastritis symptoms ,GERD also could be a cause. Next Steps Get an X-ray done. See a pulmonologist first. you will need lifestyle changes and treatment for gerd along with treatment for allergy after evaluation