In Chekov's "Gooseberries," I think that rain here symbolizes awareness.
At the start of the story, the narrator describes what Ivan (the veterinarian) and Bourkin (the school master) see around them as they spend time together and talk. Chekov specifically writes, "In the calm weather when all Nature seemed gentle and melancholy, Ivan Ivanich and Bourkin were filled with love for the fields and thought how grand and beautiful the country was." Life is good for these men; educated, they must have interesting discussions.
Bourkin reminds Ivan that he was going to tell a story the last time they spoke, indicating that they have been together recently enough that Bourkin remembers the untold tale. They are lovers of Nature, and it is the kind of day they love.
However, when Ivan prepares to begin his story, it begins to rain. As they are getting soaked, they agree to go to Aliokhin's home, which is close-by, for shelter.
Once there, Aliokhin welcomes them and suggests a bath before they visit, as it has been a long time. However, Ivan goes out into the yard and jumps into the pool of water there, swimming with the lilies. He absolutely loves the experience, and it is some time later, only with encouragement, that he is finally convinced to come out of the water. (Perhaps the swim, with the rain in his face, signifies a time when there was only beauty in the world for him, and the rain acts as reminder of the changed way in which he now views the world.)
Later the men are sitting inside and Ivan finally starts to tell his story. There is no indication that anything of consequence will be revealed in words. After all, the story is entitled, "Gooseberries."
We quickly learn that the gooseberries are a part of Ivan's brother's dream of living in a perfect place: on a farm. Ivan recounts how his brother sacrifices the things in life that are usually enjoyed daily: enough food, satisfaction with one's lot in life, and even the companionship of a spouse.
Ivan's brother brushes all of these things aside to have that farm, and loses his youth and a sense of respect and concern for others in this fantastical world he creates where he sees himself somehow bigger than life.
This all has a devastating effect on Ivan. Though we sense he is a good man, he realizes that while "happiness" may be experienced by many, how can it be complete where there are so many other people suffering quietly behind the scenes. Ivan mourns that no longer has the energy to change the world.
After the rain starts, Ivan tells his brother's story and how it affects him. Aliokhin seems unchanged by what he has heard, but Ivan can no longer be satisfied with his new awareness. He has changed: he has had a vision of what had previously been shut away from him, and as he dresses for bed, he murmurs a silent prayer that God would forgive him as a sinner.
Bourkin, the younger man of learning, has also had the truth revealed to him. He may not yet recognize what troubles him and prevents sleep from coming to him. He think of the smell of his pipe, and worries, while the rain beats all night long on the window.
Bourkin has a new awareness, and as it waits to come to the forefront of his mind, the raindrops, symbolic of awakening to the realities of the world, will not let him rest; he cannot return to his place of ignorant bliss now that Ivan has "opened his eyes."
Considering that Chekhov's paramount message in "Gooseberries," which is expressed in the lines Ivan Ivanovich speaks saying, "Don't be calm and contented! Don't let yourself be put to sleep!" directs attention to the pain, suffering and evil in life and away from the pleasures of a self-satisfied life built on the luxuries of pleasures, such as Nicolay's precious gooseberries, it is reasonable to suppose that rain symbolizes something that will reinforce this thematic direction. Some critics say the rain draws attention to the cycles of seasons and life, the coming and parting of happiness and woe, demonstrating the meaning of the line, "life will show him her laws sooner or later, trouble will come for him," but other critics dispel this reading by saying that there is no evidence in the text that the rain was intended to be understood this way.
On the other hand, some critics associate the rain with mood, that which establishes a gloomy and ominous mood in which Ivan discovers ominous truths about the happiness of some and the suffering of others. However one enlightening understanding of the rain is that it is the parallel nature emblem embodying the opposite values from those the emblematic gooseberries represent: If self-focused happiness is symbolized by the nature emblem of gooseberries, then the pain and suffering of the impoverished ones is symbolized by the nature emblem of the rain.
This understanding of the symbol of the rain is confirmed by the opening description of it and the descriptions of how nature is affected by the rain. Firstly, the opening description of the rain says, "the clouds hung low over the fields and it looks like rain, which never comes." This immediately casts the rain in a dark symbolic light because it never comes: rain that comes is associated by literary convention with relief, cleansing, hope, rebirth, but this is rain that never comes. Thus is rain symbolizes relief and hope that never comes. Secondly, the affect of the rain on nature is exemplified by how animals behave in it: Dogs tails are tucked and they stand looking mournful while horses hang their heads under the rain's pelting presence.
The affects of rain on nature confirm rain as symbolizing the law of nature that rains down misery, poverty, pain and suffering on those who are in want and in need. Rain symbolized thus forms a parallel to the symbol of the gooseberries. Further, rain as the symbol of suffering and pain exactly supports the theme of helping the needy in life instead of storing up your own happiness as a person who is calm and asleep in the face of others' suffering.