It should be noted that some Xbox Game Pass games would be cycled out
each month. Microsoft hasn’t indicated which games will be removed to
make room for the above but it did say that Game Pass titles would be up
for a long time. Regardless, take advantage of what interests you when
July 1st rolls around.

It should be noted that XBox Game Pass senior manager Dennis Ceccarelli said that

“The bigger thing is when games will leave,” he said. “We see a lot of
questions and a lot of assumptions … and while there’s not one set
amount of time that a game will be in the catalogue, and some titles
will leave the catalogue eventually, I want to put some fears at rest
because our first games won’t be leaving the catalogue until late
November.

Source: http://www.eurogamer.net/articles/2017-06-15-there-will-be-at-least-five-new-xbox-game-pass-games-every-month