The preposition plus has long been used with a meaning equivalent to and (as in "two plus two"); people have come to use plus as a conjunction much like and. Sense 2 is considered to be an adverb by some. It is used chiefly in speech and in informal writing.
First of all, you already did log in to GPT plus.
The proof is that you can chat with GPT4 using the web UI which is only available to plus subscribers.
It appears you were looking for a very specific naming scheme and to be absolutely clear:
You log in to chat.openai.com using your OpenAI account and if you are subscribed to Plus then the service will immediately work as expected without any further action needed.