Can baby teething cause gas, discomfort, constipation, lack of appetite?

No. Teething has been wrongfully blamed for the common maladies in infants of 6 to 12 months age such as fever (febrile illness), diarrhea or even rash. It would be reasonable to expect a mild elevation of temperature not above 101 degree F and some fretfullness.
Teething causes none. Teething causes none of the above symptoms. The constipation can can, indeed, result in cramps, excessive gas and ack of appetite, but not teething.
No. Physiologically there is no reason for teething to be painful or cause any of the symptoms that you are asking about.
Not really. More likely it's a GI problem. Check w Pediatrician. See Pediatric Dental Specialist for 1st Dental Exam Age 1.