The National Screen Service was a company that provided advertising materials, including movie posters, to movie theaters in the United States from the early 1900s until the early 2000s. Their logo on a movie poster is an indication that the poster was distributed by them to movie theaters for promotional purposes.
Graphics United America, on the other hand, was a company that specialized in designing and printing movie posters, among other advertising materials. Their logo on a movie poster indicates that they were responsible for creating the poster, and it was likely distributed by them as well.
Having a NSS or GUA logo on an original movie poster can also indicate the authenticity and age of the poster, as these logos were typically used in the mid-20th century and earlier. Many collectors and enthusiasts value original movie posters with these logos as historical artifacts and rare pieces of movie memorabilia.