As far as I am aware, GregTech changes
- A lot of IC², and as an indirect result, anything that uses IC²
- BuildCraft quarry recipe
- Forestry bronze output
- Log -> planks and planks -> sticks recipes, if and only if they use oredict logs/planks with a specific naming convention
- At one time, and disabled by default, had an alternate recipe for RP2 block breakers (which used purely vanilla inputs, so it is very easy to do even without ever downloading RP2).
- Ingot/gem <-> block recipes, if configured to do so
If the block, log, quarry, and bronze changes are disabled, then GregTech only alters a few vanilla recipes (also configurable), IC² (somewhat or entirely configurable), and mods that use IC² components and machines. Unless someone can point out anything else that GregTech changes? (other than mods like Modular Powersuits that add their own GregTech-based recipes)
Also, you might be interested that GregTech allows you to adjust the default item despawn time, which could even be a *buff* (vanilla default is 5 minutes. In the GregTech config, you can easily set that to any number of ticks. Try an hour, and see how it alters the gameplay)