In order to accurately summarize "The Garden Party" in a way that is as minimalistic as possible, you must take into consideration at least the four most important and sequential actions that help start, mold, and end the story.
Looking back at the story, the basic events can be easily re-told. Laura Sheridan's family's garden party is almost interrupted by news of the death of the local carter. This catalyst makes naïve Laura see life as a combination of joy and sadness. Her reaction represents her transformation, from shallow and immature, into responsible and sensitive. Laura's change is complete when she visits the dead carter's home and, as she sees his beautiful body laid ready to be buried, an overwhelming sensation of discovery makes her utter her new view of life, however, she is left speechless
This particular suggestion may be more than 60 words, but if you leave out the details that sort of decorate the actual account of the four major events you can actually get a story as synthesized as can possibly be without losing the central theme of it. In the case of "The Garden Party", it is important to maintain the atmosphere of change, awe, and coming-of-age maturation that Mansfield certainly intends to give her story.
Hence, your four major points are a) the garden party b) the news of the carter's death, c) Laura's state of mind after learning the news of the death, d) Laura's final state of mind after she sees the deceased body of the carter. Use those four points as your frame and tell what happens in each of this point with clarity, and avoiding too many adjectives.