Divide, round and replace in one select statement - tranformation from bytes to GB in SQL Server
The query returns a float data type, which is a binary structure that has no comma or dot decimal separator. It is the rendering application that converts the value into a string format for display purposes, which can honor the client's regional settings if programmed to do so.
Although you could change the T-SQL to return a formatted string like the example below, be aware client sorting will use string instead of numeric rules. Note this example explicitly specifies culture en_US, which uses a dot decimal separator, to override the current session language setting. Also added ORDER BY for deterministic results with the TOP clause.
SELECT TOP (1000) 
      [Sharename]
      ,[ShareSize]
      ,FORMAT(ShareSize / 1e+9, '#,##0.000000000000', 'en-US') SharesizeGB
FROM [mydb].[dbo].[myshares]
ORDER BY (ShareSize / 1e+9) DESC;