ぐらい is a colloquial form of くらい.

• amount + ぐらい = about, approximately
本を十冊ぐらい 買いました。(I) bought about ten books.
 
• noun + ぐらい + negative = as much as . . . .
あの人ぐらいのんびりした人も少ない。
Not many people are as laid-back as that person.
 
• noun + ぐらい = at least
お茶ぐらいゆっくり飲ませてください。
Let me at least drink tea leisurely.
 

• verb-plain + ぐらい = as . . . as . . . , to that extent, to that degree
涙が出るぐらい 笑ってしまった。(I) laughed until tears came out.
電話しようと思ったくらいです。I almost telephoned you.
 

• adjective + くらい = almost . . .
まだ九月なのに、もう寒いくらいです。It's still September, but it's almost cold.

 

• In the "amount + ぐらい + positive" pattern, ぐらい may be replaced by ほど or ばかり。 Go to hodo (ほど)    Go to bakari (ばかり)
この仕事は一週間ぐらいかかります。This work takes about a week.
この仕事は一週間ほどかかります。This work takes about a week.
この仕事は一週間 ばかりかかります。This work takes about a week.