Helloooo
Well, the way I see it is this:
~ごろ(に) = at about
~ぐらい = (for/in) about
But, honestly, I'm not sure if they're interchangeable, but this is the way I use them:
九時に起きました。(ku ji ni okimashita.)
I got up at 9:00.
九時ごろ起きました。(ku ji goro okimashita.)
I got up at about 9:00.
and
マクドナルドの前で一時間待ちました。(makudonarudo no mae de ichi jikan machimashita.)
I waited in front of MacDonald's for one hour.
マクドナルドの前で一時間ぐらい待ちました。(makudonarudo no mae de ichi ji kan gurai machimashita,)
I waited in front of MacDonald's for about one hour.
Hope this helps
Oh, by the way, I just looked in my book, and when it made a side note about ぐらい, it said "for "at about a certain time", we have another word ごろ". So, maybe they are the same, or interchangeable.
-Tommy
EDIT:
I changed the wrong particle ^^;;