Separation token in GPT for text similarity/question answering

What is the separation token used to separate two input sequences for GPT?

Given a pretrained GPT2, I’m interested in fine tuning the task for question answering (given a string of question, and a string of answer, need to classify 1 or 0, whether it is indeed the answer to the question or not)

In BERT, the input sequences are separated with a [SEP] token, and this classification can be done by feeding in once sequence: question_text [SEP] answer_text

What is the separation token in GPT required for this? If i’m finetuning a pretrained model, it means that this separation token would not have been encountered before, so will i be able to use any token i wish for this? and the mode would just learn that to be the token

GPT2 don’t have anything like SEP. All it have is or something similar.

Maybe an example would answer the question?