Try Pianoteq Stage for Mac or Windows. It is exactly what you want. It costs €99 or US $129. Using a physical-modeling synthesizer, not samples, it really sounds like a grand piano -- in fact, you can choose between several different kinds of grand pianos.
You can download a free trial version.
If you buy the more expensive Pianoteq Standard or Pianoteq Pro products, they permit you to modify many parameters of the piano sound to tailor the sound to your needs.
I must stress, though, Pianoteq is a very specialized product -- a uniquely-designed digital synthesizer built from the ground up to emulate piano sounds, with a proprietary design.
You can't make a convincing acoustic grand piano sound by manipulating parameters in any other kind of general-purpose synthesizer, in hardware or software.
The Roland and Yamaha companies each make a hardware synthesizer-keyboard (called a "virtual piano") that emulates an acoustic piano without using samples, like what Pianoteq does. But these particular Roland and Yamaha keyboards cost US$6,000 and up.
Pianoteq runs on just about any Mac or Windows computer (Linux, too) and it's quite affordable. Unlike grand-piano sample-based virtual instruments, it doesn't require gigabytes of hard drive space, a large amount of RAM, or a really fast computer to work correctly.