There are no differences between DS3150TN+ and DS3150TN+GSW.
The GSW suffix is a generic special for one customer only. As these parts are on Last Time Buy (LTB), this customer has set aside a large amount of parts which they promise to purchase over time. No one else can purchase these "GSW" parts as they are, therefore, spoken for.
There is no technical difference in the specifications, performance, package between parts with and/or without the GSW suffix.