All RCT games are updated and tweaked to run on modern OS.

No, don't worry. Unlike Steam, GOG actually cares about getting games to run.

GOG offers all its games patched with the last official patch included. If a game doesn't run out of the box in this state, they add technical tweaks like tweaked launchers, alternative driver link libraries or a DOSBox or SCUMMVM environment to make them compatible with modern versions of Windows and Mac OS. Nothing more, nothing less.

Regarding Rollercoaster Tycoon Deluxe, GOG added an additional executable file called launcher.exe. It automatically kills explorer.exe to circumvent a color palette incompatibility on newer Windows and then launches the game, starting explorer again upon exit. Alternatively you can use the original executable to start the game, as the "weird colors" bug doesn't concern all systems.

From my observations the Steam versions rarely get updates while GOG maintains these to work. Not to smack any devs here, but since we're talking Atari these days the probability may be lower.

Also I hear that the mac version of RCT3 is the same as running it in wineskin (which is actually pretty funny considering it's in the official app store as well), if this is true there's really nothing to worry about either as the versions here can be made to run on such pretty easily.