Did they target next gen for this? What makes it look better than Gears 5?
If you're comparing Gears 5 on Xbox One X to it on Series X, they bumped it up to PC Ultra settings and beyond on the latter. If you already played Gears 5 maxed out on PC, the jump is much less significant, though they've added the new effects they did for Series X to the PC version now as well—some improved lighting and shadow effects, basically. (All these improvements apply to Gears 5 in general and not just Hivebusters.)
Can you install this without installing gears 5? that game is too big
They added an install manager to Gears 5, so you can actually do a base install (~40 GB, seems to include shared assets and the first act of the campaign), then launch the game and pick what other parts you want to install. I only have that base install and Hivebusters installed (on PC Game Pass).
Does this have screen space GI on PC?
Yes, they've added that now, along with all the other Series X effects as far as I can tell.
How do I access this if I bought Gears 5 and have GP ultimate? Do I automatically get it?
You'd have free access to this through Game Pass Ultimate. Exactly what menus you'd have to jump through to get it depends on your platform, but I expect you could just launch Gears 5 and follow any in-game menus that advertise Hivebusters.
Screenshots are amazing, is this like Uncharted Lost Legacy, or an actual DLC?
If you mean in terms of how it's delivered, as a fully stand-alone thing or DLC add-on, it's the latter. You'd need to have Gears 5 already.