Gastritis does not cause mucous in stools. From your history there is no cause which is explaining mucous in stools.
Various causes are possible for this.
IBS COLITIS PILES POLYPS LIST IS LONG.
ALL OF these have different Treatment.
Discuss in details about your SYMPTOMS.
Next Steps
avoid spicy foods coffee tea cold drinks alcohol smoking junk food etc.
Health Tips
diet and lifestyle changes can help SIGNIFICANTLY.