How to set a new players data to something?

All you have to do is use the same thing you used for coins and gems. You need to use GetAsync in order to get the values, and check if they've been set.

As the wiki states, "If the key does not exist, [GetAsync] returns nil." Using this information, you can determine whether or not they've played the game before, and set it to either the saved value, or "Default".

local DS1 = game:GetService("DataStoreService"):GetDataStore("Coins")
local DS2 = game:GetService("DataStoreService"):GetDataStore("Gems")
local DS3 = game:GetService("DataStoreService"):GetDataStore("equippedKnife")
local DS4 = game:GetService("DataStoreService"):GetDataStore("equippedGun")

game.Players.PlayerAdded:connect(function(player)
    local id = player.UserId;
    
    local stats = Instance.new("IntValue", player)
    stats.Name = "leaderstats"

    local coins = Instance.new("IntValue", stats)
    coins.Name = "Coins"
    coins.Value = DS1:GetAsync(id.."_DS1") --// You need to change this to be the same as the key you saved it as.

    local gems = Instance.new("IntValue", stats)
    gems.Name = "Gems"
    gems.Value = DS2:GetAsync(id.."_DS2") --// Same with this.

    local inGame = Instance.new("BoolValue", player)
    inGame.Name = "InGame"

    local equippedKnife = Instance.new ("StringValue", player)
    equippedKnife.Name = "equippedKnife"
    equippedKnife.Value = DS3:GetAsync(id.."_DS3") or "Default" --// Get the value, and fallback to "Default" if it doesn't exist.

    local equippedGun = Instance.new("StringValue", player)
    equippedGun.Name = "equippedGun"
    equippedGun.Value = DS4:GetAsync(id.."_DS4") or "Default" --// Same as above.

    while wait(60) do
        DS1:SetAsync(player.UserId.."_DS1", coins.Value)
        print("Saved Coins")
        --Saved Coins
        DS2:SetAsync(player.UserId.."_DS2", gems.Value)
        print ("Saved Gems")
        --Saved Gems
        DS3:SetAsync(player.UserId.."_DS3", equippedKnife.Value)
        print ("Saved knifeEquipped")
        --Saved equipped Knife
        DS4:SetAsync(player.UserId.."_DS4", equippedGun.Value)
        print ("Saved gunEquipped")
        --Saved equipped Gun
      end
end)

You may want to look into using a table to save values as that may be a bit neater, but this way would work fine as well.
Hope this helped.