You use `an` in front of an acronym when the first letter of it has a vowel _sound_, even if the letter itself is not a vowel.

Example: "A member of parliament talked to me." "An MP talked to me."

Here when pronouncing the m in `MP` the vowel sound for e is made, as in 'em'.

As the poster commented, it was a typo in this case and should have been "a". But, in general, we would use "an" preceding an acronym whenever the pronunciation of the acronym begins with a vowel sound. To use some broadcast networks as examples, you could say "A CBS production" [analogous to "a CGI"], but "An ABC production" or "An NBC production" (note NBC is pronounced starting with a vowel sound, at the start of "en").