15
Each one of those letters represents a nucleotide. So to find out how many nucleotides are in the gene, just count them
However, keep in mind that this only represents 1/2 strands that make up DNA. If you are supposed to include the other strand, simply double 15, and there would be 30 nucleotides total. However, only 1 strand is actually required in order for protein synthesis to work, so I think it's just 15.