GBIC stands for Gigabit Interface Converter, and it allows a way of linking to different media, such as fiber optic cables. SFP stands for Small Form Factor Pluggable, although, GBIC and SFP both have the same purpose. The significant difference between GBIC and SFP is their size. SFP is relatively smaller than GBIC.
SFP has gained popularity over GBIC because it lets you put more than one rack unit. It is because of this that GBIC is now considered outdated. It is not very likely that you would find suppliers that still carry equipment that is compatible with GBIC. Also, you would have to spend the additional money to acquire equipment corresponding with GBIC, while SFP has set the new standards. That being said, both GBIC and SFP are equal in performance.