ok i don't know if you guys can read or not but the question said 'new ip' not new half life or new portal game. this means there'll be something completely new set in the half life universe.
also the question said 'chance of a new ip' and since gabe newell is a businessman who probably knows the dos and don'ts of answering questions he'll probably answer everything as vaguely as possible just to get a positive reaction and build up hype and interest in valve as a whole
then again i doubt valve will ever release anything considering their employees can chose to work on whatever they want and if they lose interest they'll just stop working on whatever new game they're making. also don't forget valve has scrapped games to remake them better in the past and i bet you millions that they scrapped games but lost interest in making it better