I assume you are referring to the LX/LH GBIC (WS-G5486).
The LX/LH GBIC is specified to operate up to 550m over multimode fiber (both 50-micron and 62.5-micron diameter strands). At distances beyond 250m, and Cisco says 300m, a mode-conditioning launch cable is required.
The LX/LH GBIC is also rated to operate up to 10km over single-mode fiber. (5km is the standard distance.) Regular single-mode patch cables are all that's needed. The performance characteristics of the optical elements are such that you will not overdrive the receive optics with the maximum optical power output from the transmit side. Therefore, in-line attenuators are not needed.
The actual distance you are able to go is primarily dependant on the end-to-end attenuation of the fiber span. This includes connector loss, splice loss, and cable attenuation as a function of distance. I have heard of people running 600 to 700m on 62.5-micron multimode OK (50-micron fiber is better if you're going to push it); and I have run LX/LH GBICs out to 16km over single-mode without problem.
The minimum optical power budget for these GBICs is 9.5dB. If you can, test your fiber cable end-to-end with an optical power meter, measuring at the 1300nm wavelength. If the loss is less than 9.5dB, you should be fine.
Hope this helps.
