How to import large ASCII into GIS (Grav/Mag) data?

Geophysical data files are normally distributed as point-located data. That is, they give various measurements at a specific point. You would need to import this as a vector point file and then use gridding techniques to arrive at an image for each variable, if that is what you want. There are many processing techniques for geophysical data. See for example this excellent book: https://www.aseg.org.au/sites/default/files/ebook-ASEG-eBook-Geo-Interpretation-of-Aeromagnetic-Data-Opt-for-user-1621.pdf

Having said that, you need to know the file structure. The example you give is gravity data. Are the entries comma-delimited as your example suggests? If not, you need to establish the delimiting character. It looks like you have no header line so that information might be in a different file (organizations use different formats and there is typically a description file as well as the data files, so you might need to go back to the source).

CSV imports of point vector data are straightforward. You can use either the X,Y or the Lat,Long depending on whether you want the final data in a projected coordinate system. Typically the X and Y would be UTM coordinates for the appropriate zone, while the Lat-Long would be WGS84 as captured by the GPS system. Using Lat-Long is safer because the X-Y might be a local grid.

From what you say, you may be encountering an error related to a maximum size for a CSV file import. I think there is a limit but I'm not certain. Someone else will know and be able to tell us what it is.

Here is a useful link for geophysical data formats: https://www.aseg.org.au/sites/default/files/pdf/ASEG-GDF2-REV4.pdf

Your data structure might be different but this should give some guidance.