(Disclaimer: I'm no SQL Server expert, but I'll give it a go)
SQL Server is case sensitive - you have to quote your column names, so instead of
SELECT COLUMN_NAME FROM TABLE@mssql
you need
SELECT "COLUMN_NAME" FROM TABLE@mssql
or even
SELECT "COLUMN_NAME" FROM "TABLE"@mssql
See Oracle forums on SQL Server, Oracle Gateway and ORA-00904