I'm having a trouble following the thrust of the question because the adventitia of the esophagus and the subserosa of the stomach are not analogous.
The adventitia and serosa are the outermost layers of the esophagus and stomach, respectively. If the tumor invades either one, it's a T3 tumor. If the tumor falls short of reaching this outermost layer, then the tumor only is classified as invading the muscularis propria (in the case of the esophageal primary) or as invading the muscularis propria or subserosa (in the case of the gastric primary); hence, T2. In either case, T4 signifies invasion into adjacent structures...doesn't apply here.
In any case, I would consider your situation to represent an esophageal rather than a gastric primary. But yeah, it's tricky since I assume you're talking about the anatomic GE junction rather than the squamocolumnar junction (oh wait, you said not associated with Barrett's...nevermind). But let's go through the following mental exercise:
The actual stage (based on how the T, N, and M classifications) determines treatment. So a T3 N1 M0 esophageal primary is a Stage III cancer. A T2 N2 M0 gastric primary is a Stage III cancer as well. The prognosis in either case is not good. Treatment probably doesn't differ by all that much. Patient probably gets chemo and radiation therapy.