Giga base or Giga byte
This refers to base pairs.

File size has no particular meaning beyond practical considerations, given that it depends on the format. (For example, 2bit files use 2 bits per base, as the name indicates, compared to the 8 bits necessary for each letter in a plain text format like FASTA &amp; derivatives.)

For your additional question, (number of reads * length of each read) gives the total length of sequence contained in the file. It is not really a property of the file format, but a parameter of the sequencing run.