Metamorphic testing (MT) is an established testing methodology suitable for testing various types of systems. While performing MT, software testers face the challenge of identifying and implementing metamorphic relations (MRs) for their software systems. This paper introduces GeMTest, a general-purpose metamorphic testing framework that is domain-independent, enabling software testers to implement MRs in Python and execute them with pytest. The implementation of MRs is done by annotating Python functions, which implement the follow-up generation function, the metamorphic oracle, and the system under test with decorators provided by GeMTest. This allows GeMTest to automatically create and execute a pytest test suite, containing multiple metamorphic test cases derived from the user-defined MRs. We evaluate GeMTest by implementing 218 MRs from 16 program domains, ranging from SAT solvers to deep learning image classifiers. To enable the adoption and encourage further extension, an open-source implementation of GeMTest is available. Our demo video is available at https://youtu.be/Ec5SK-meu90.