KINDA ODD.........
That the Rust is All on One Side. The Other Side is All Bright and Perky. Like it Should Be.
You might consider that fact to be a clue and try to determine if there's anything in your storage modes that might be causing the rust.
Were it mine [and it isn't], I'd take a Q-tip moistened with a bit of phosphoric acid and rub it on the rust. Typically, the serious rust then turns black [iron oxide converts to ferric phosphate] and flakes off. Naval Jelly is a phosphoric acid treatment. The base metal is then usually left in clean condition and more resistant to rust than previously, due to a slight phosphate coating.
And then, I'd shoot that puppy until it breaks. If it ever does. [It's a Glock.] And then send it back to the factory for repair, if you are still sentimental about it.
Better yet, join Glock Shooting Sports Foundation and go to Glock Armorer's School. Get your annual Glock purchase discount and buy another one. From Chris Z; he's a Fair and Honest Broker. And another one. [Friend of Chris' has One of Each.]
Or.....Get a Glock in .357Sig. Buy Wolf or Storm Lake .40S&W and 9mm [plus extractor and magazines] drop-in barrels.
Shoot everything under the sun. Win another Glock at Rock-Your-Glock.