If it were me, I’d forego the purchase of a Shimano 105 GS rear derailleur and instead purchase a far more capable Shimano mountain bike 9-speed rear derailleur.

The Shimano 105 GS rear derailleur can only handle (officially) a max rear cog size of 28 teeth and has a chain wrap capacity of 39.

Conversely, all Shimano mountain bike 9-speed rear derailleurs can handle a max rear cog size of 34 teeth (36 teeth on certain models) and have a chain wrap capacity of at least 43 to 45.

A Shimano 105 GS rear derailleur came on my Trek 2.1 road bike triple and I’d replace it with a Shimano 9-speed mountain bike rear derailleur in a heartbeat if I needed to install an 11-32 or 12-32 10-speed cassette in place of my 11-28 10-speed cassette. I’m not impressed in the least of ways by the Shimano 105 group.