Yeah I'd recommend you try other dictionaries like "Y gweiadur" or "Ap geiriaduron". GPC, is a great dictionary but it's more for first language/fluent speakers. If you can't find a word else where then it is worth looking up GPC, but just remember that it's got a lot of different varieties of words some of which are old fashioned or really formal.
I think this is the issue between North American and British English. North Americans very rarely use the word "toilet" unless they're referring to the actual porceline fixture. I'd go as far as to say it's generally considered uncouth to say that you are "going to the toilet" (unless you were specifically discussing bodiky functions). It's much more common to say that you are "going to the bathroom, even if you have no intention of bathing. So, it might be a good idea to allow" bathroom for tŷ bach.