I just used the in-game settings to switch to windowed mode, and I double clicked on the window title area, and it maximized the window to my widescreen laptop display! The game now looks much smoother (not pixelated) and it's really widescreen!! Just awesome!! (I did not modify the game in any way, I use plain vanilla GOG RCT, as installed, that I recently bought).
There's a limit to that too, so I suppose your laptop is rather low-res?
1280x768. It's a 10" display. I don't complain, the game looks great now. It was fun in fullscreen pixelated 4:3 aspect and now it's even better! The hardware is great to play all these retro games, and even newer ones.
If you have a higher resolution display, than you'll probably want to use OpenRCT2. This is a retro game after all, but a real gem and a classic! You have to take into account the games capabilities, and accept it for what it is.. It's a masterpiece in it's own right. If you want better graphics, there are newer games, let's not get carried away. Even OpenRCT2 has limits - and that's why I bought on GOG the original, to see what it's like (and it's really, really great, though I'll go on to OpenRCT2 when I'm done).
Also, according to some reviews, the GOG version is different than other retail versions that people bought in the past. Using the hexeditor trick did not work for me as well and it's a crazy amount of work for many, like some other more exotic tricks, which also don't work for everyone. What I found out by accident is quick and simple, and it works for me fine. Hopefully, other people will find it helpful, I imagine that many people with average hardware specs play this, it's a great game for mobile devices.
And if it does not work for you, because you have a too high resolution display - get a low resolution display!