The reason to use an attention line is to direct your letter to a specific person within an organization while wanting the letter to be read by the company as a whole. So Gentlemen is used in the salutation
For example
ABC Marketing Services
123 Someplace Street
Somewhere, Manitoba
R3C 2M7
Attention: Mr. John Q. Public, General Manager
Gentlemen:
If the letter is being directed specifically to one person in the organization the letter would start this way
Mr. John Q. Public, General Manager
ABC Marketing Services
123 Someplace Street
Somewhere, Manitoba
R3C 2M7
Dear Sir: