I recently got a poster scanned that sized over 218 mb, it was really a huge size. I discovered that while scanning it had pixels with CMYK 4 color process colors intact. So my question is that, is there any way to get rid of them, and convert them into RGB?
That is actually a bit tricky. As you already pointed out, offset printing is done with applying at least the four base colors CMYK (cyan, magenta, yellow and black) in a grid, called a halftone pattern. The 4 grids are rotated just so much as that they do not form a moire pattern. When you scan or photograph such an image with enough resolution, you will see this pattern.
How to get rid of it
The only way, I am aware of, if to apply gaussian blur just enough that the pattern becomes invisible (and some of the details as well) and then trying to rescue as much detail via applying unsharp masking.
Please note that you will lose some detail this way.
"Descreening" can be done quite well in the frequency domain (using a fast Fourier transform (FFT)). This article gives an overview of the technique. G'MIC is a plugin suite for GIMP providing FFT functionality, among many other features.
EDIT: @xiota mentioned another answer in the comments describing the process in depth.