Buying GM through Steam is a rather big no-no as YoYo in unaware of the purchases so when it comes to upgrades and offer through their site (which happen a bit more often than big steam sales, and tend to be a bit better as well) you can't take advantage of them.
I've experience with this - you can just email Yoyo with your steam purchase and they'll send through your upgrade details - happened to me before :) It's a bit of a pain but overall the results are the same.
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Buying GM through Steam is a rather big no-no as YoYo in unaware of the purchases so when it comes to upgrades and offer through their site (which happen a bit more often than big steam sales, and tend to be a bit better as well) you can't take advantage of them.