Data Retrival from the GenBank?
I think that using NCBI Gene would be a better option here:


Go to NCBI Gene http://www.ncbi.nlm.nih.gov/gene/
Enter your gene of interest, in this case CNGA3 (and optionally the species)
Click the species of interest (I'm assuming you are looking for human, the first search result)
On the information page scroll down and click on FASTA which will provide the sequence for the gene of interest.