How do the gems work exactly that make you think Valve is depopulating their market?
In order to get gems you break down items in your inventory. Items in your inventory include emoticons, cards, and the like.
By having items with value (there is a market place) traded for a non-cash currency, you can get people to convert non-cash rewards back into another non-cash currency. Converting from items to gems is a sink of resources. Valve makes this clear because it takes about 750 gems to make a 3 card pack, but those three cards sell for 75 total gems. Utilizing this, people eliminate items from inventory, spend real money on items to convert into gems, and actually wind up with less goods than they initially had. Valve is getting people to inflate the value of goods, by decreasing the quantity of them.
Looking at one example.
1) I have 20 items in my inventory, which can be converted into 750 gems. Valve has set the conversion to this value, but it can be modified dynamically based on total inventory of items available.
2) I create a card pack for my 750 gems, because I cannot possibly afford a game at that price.
3) I get three cards, which trade for 25 gems a piece.
4) I have converted 750 gems into 75, a 90% reduction in electronic goods.
5) If each user takes their existing item inventory and does this process with 80% of it twice, you have 0.8% of the initial inventory remaining.
6) If only 20.8% of the inventories exist, then each item will quintuple in value to make up for the scarcity of the resources.
7) Valve dynamically drives that 90% reduction, until the average cost of everything is where they want it, which is easy because they dynamically track inventory.
8) Valve drives up market place prices, which increases their cut on transactions. Said transactions are permanently inflated (with a dynamic control system of gem conversion), thus making Valve more money consistently.
9) Have customers bid against one another, creating a competition where gem value is depleted. Those who "win" the "free" games have spent real money without knowing it, because it is two-three step removed. They feel like the game is free, while still paying real money for it.
10) All of the games "given" away are paid for in a month (or less) by the increased profits on market transactions. Everything after this is just money in the bank for Valve.
Hopefully that explain my point simply enough. If you want stats, look at card values. Steam tracks the average sales price over time, and there has been a distinct spike in values since the 15th.