Although I was not able to find whether or not there is any restriction on performing such scans in an office setting, I was able to find that prior to performing gastric emptying scan, a prior authorization should be obtained and medical necessity should be proved. There are some indications for which 78264 is a covered diagnosis and coverage will not be provided if the diagnosis is not covered. Some of the covered diagnosis include gastroparesis (symptoms), dumping syndrome, gastric outlet obstruction, dysphagia, incontinence of feces, intestinal obstruction, dyspepsia, etc.
Also, although I was not able to find whether or not a gastroenterologist can perform the study, what I was able to find was that the use of radiopharmaceuticals is strictly governed by guidelines and is regulated by the Nuclear Regulatory Commission (NRC). A person administering radiopharmaceuticals should have either a license from the NRC or be credentialed by an institution having a broad license from the NRC. So, unless the gastroenterologist has these credentials, she cannot be performing the scan.
Along with reporting the procedure with 78264, the appropriate code for the supply of the radiopharmaceutical should also be used. For gastric emptying scans, the radiopharmaceutical used is Technetium Tc-99m Sulfur Colloid, Diagnostic reported using A9541 or Indium -111 Diethylenetriamine Pentaacetic acid (DTPA) reported using A4641. Also, 78264 has a TC and professional component --- so if the gastroenterologist is only providing the interpretation for the procedure, the code can be reported using the modifier 26.