Is the breakfast in the hotel worth the price quoted ?
I have no idea about the breakfast but I would like to tell you few things.
Location is good but room too small and even bathroom.
You will feel suffocated
I've stayed there with my 6yo daughter, my Japanese brother and his partner for 3 nights last year.
Their breakfast was Japanese and Western style buffet. I'm a vegetarian but enjoyed variety of choice, so as my daughter did (btw their miso soup was quite salty). However, my brother, who often stays that type of hotels, told me that it didn't worth for money as there're many Japanese hotels serve more quality dishes. Saw a couple of same dishes every morning, especially Western dishes.
There're many convenience stores around the hotel (at the first floor and the next bldg) and the stn, so it would be an option for you to buy some food from there.
Sorry for my late answer if you've already gone to Tokyo.
Get an answer from Hotel MyStays Gotanda Station staff and past guests.