If they ask you, you can't lie, and you can't obfuscate. If you're in secondary screening and they're asking you "Who precisely are you visiting and what is your relationship?" obviously you're going to have to say girlfriend. You should never be lying. Lying will get you banned from Canada.
What @mau123 is talking about is primary inspection: "Why are you visiting Canada?" "Tourism, going to see Niagara Falls and Toronto." not "I'm here to visit my girlfriend, and I earlier applied for an IEC to be with her more but was refused because I missed my biometrics deadline." and not "I'm not seeing anyone here. I don't know anyone in Canada at all."
But if they don't pull you into secondary, don't worry.
What @mau123 is talking about is primary inspection: "Why are you visiting Canada?" "Tourism, going to see Niagara Falls and Toronto." not "I'm here to visit my girlfriend, and I earlier applied for an IEC to be with her more but was refused because I missed my biometrics deadline." and not "I'm not seeing anyone here. I don't know anyone in Canada at all."
But if they don't pull you into secondary, don't worry.