If you have an abdominal ultrasound for gallbladder, can it also pick up an aortic abdominal aneurysm?

Maybe. Sometimes seen but need ultrasound of aorta or retroperitoneum to evaluate aorta formally.
Possibly. Ultrasounds are excellent tests to look for abdominal aortic aneurysms yet the ultrasound technician would not specifically look for an aneurysm unless the test was ordered for that purpose. If your doctor ordered an ultrasound of the gallbladder, this test would be limited to the gallbladder, liver, +/- pancreas.