A gigaelectonvolt is 1 billion electronvolts.
The prefix "giga" means 10 to the power 9; the prefix "exa" means 10 to the power 18.
Your question contains mixed units. billion is not an SI prefix, its numerical place as a multiplier is taken by Giga, 10 to the power 9. Peta is 10**15, and Exa is 10**18. [** means to the power of.] with the new proper units, you may do the math.