I am developing an Android application in which I need to show how many mails the user has not replied to (based on a particular sender/subject).
Is this is feasible?
I haven't particularly dabbled in GMail API but as per googling and looking around the community for a while, I think it seems feasible. I think you could make use of labels and threads. As per threads docs:
The Gmail API uses Thread resources to group email replies with their original message into a single conversation or thread. This allows you to retrieve all messages in a conversation, in order, making it easier to have context for a message or to refine search results.
I think you can just tweak around it, like to get the most recent message in a thread, then check if it was sent by the user, then just return true or false as the identifier. Here's a post that's about retrieving the reply of a message, might be useful. Code and logic is still up to you though.