Question: how is gsm different from nfa and dfa?
Answer: An NFA is a Nondeterministic Finite Automaton. Nondeterministic means it can transition to, and be in, multiple states at once (i.e. for some given input). A DFA is a Deterministic Finite Automaton. Deterministic means that it can only be in, and transition to, one state at a time (i.e. for some given input).