Glandular epithelium is developed from covering epithelium

Mechanism of formation of exocrine glands from covering epithelium
epithelial tissue broadly classify into two group
1. covering epithelium
2. glandular epithelium
Glandular epithelium with duct is known as exocrine glands
Glandular epithelium without duct is known as endocrine glands
Both exocrine and endocrine glands are developed from covering epithelium
exocrine glands remain their contact with covering epithelium but endocrine glands lost the contact .

Gastrointestinal tract related with different exocrine glands.
in generally, there are four histological layers of gastrointestinal tract
1. mucosa : is formed by  three layers
a) lining epithelium
b) lamina propria : connective tissue
c) muscularis mucosae: smooth muscle cells
2. submucosa : connective tissue
3. muscles layer: smooth muscle cells
4. serosa : formed by connective tissue and simple squamous epithelium (mesothelium)
From the lining epithelium of gastrointestinal tract glandular epithelium begin to form
some exocrine gland enter into lamina propria ex: gastric glands, intestinal gland etc
some are enter into submucosa ex: Brunner;s gland
some are go out side of gut wall ex: liver, pancreas
So, all glandular epithelium is originate from covering epithelium