ごろ、is only used for time, date, seasons, 
3時ごろ about 3 o'clock、秋ごろ、around this autumn

ぐらい、You can use for many things like
I am about 50 kg. 50キロぐらい
I swam about 100 M.　　100メートルぐらい
I will go there around 3.　3時ぐらい

This is just extra info. 

Young children usually use onlyぐらい. Because young children don't have clear idea of time. They can't use ごろfor time.

You can use both for time. But if you use ぐらいfor time in formal situation, it might sound strange. 

In business scene,

社長は５時ぐらいに来られます。←It sounds strange. 　

社長は5時ごろに来られます。← better