Strike Suit Infinity is basically standalone DLC, at a DLC price. I don't see any disadvantage to it being available to those who don't own Strike Suit Zero, we just wanted people to be able to play it without needing the original game (and who knows, maybe after playing Infinity they'd be interested in buying Zero).
There's actually a fair amount of content, and a bit more to it than just killing waves. It's split up into 18 unique rounds (which each consist of a number of waves), and you earn credits to spend on reinforcements to bring with you into the following round so there's a tactical element to it. You also get access to the two DLC Strike Suits (Raptor and Marauder), and it has some new enemies exclusive to Infinity.
We hadn't initially planned a GOG release because it's heavily integrated with the Steam leaderboards. In fact, we're not even sure if GOG themselves would be too keen on letting us release a leaderboards-based game that has the leaderboards stripped out...
If it's quality content, I can live without leaderboards. Maybe knock the price down a dollar or two for the GOG version to make up for the lack of that feature. We chose to purchase Strike Suit Zero here for a reason and we shouldn't have to go elsewhere for the rest of the content, standalone or not.