'Twas only a guess.
This is what I found about Schwannoma when I did a search:
"Neoplasm of uncertain behavior of endocrine glands and nervous system."
So in this case it is a neoplasm in the stomach. Which one is it? Gastric, endocrine or nervous? Or all of the above?
V12.40 is "Unspecified disorder of nervous system and sense organs"
while
V12.49 is "Other disorders of nervous system and sense organs"
I also found this:
Gastric schwannomas, the most common GI schwannoma, account only for 0.2% of all gastric tumors, and principally involve the submucosa and muscularis propria [4–6]. They grow slowly and exophytically and are usually asymptomatic. Because of this indolent growth pattern, as with our case, these tumors often discovered incidentally via cross-sectional imaging or endoscopy [8, 11]. If symptomatic, the most common presenting symptom is upper GI bleeding, which may be secondary to the growing submucosal mass compromising the blood supply to the overlying mucosa. The mucosa overlying the mass may then ulcerate secondary to ischemia or from a reduced tolerance to the gastric acidity [4, 10, 11].
From:
Case Reports in Surgery
Volume 2012 (2012), Article ID 280982, 5 pages
doi:10.1155/2012/280982
So really it might start as a neoplasm on a nerve I guess but it becomes a gastric tumor. So I would still go with V12.79 since we don't know if it was benign or malignant.