GDAL supports reading and writing ASCII gridded XYZ raster datasets (i.e. ungridded XYZ, LIDAR XYZ etc. must be opened by other means. See the documentation of the gdal_grid utility).
Those datasets are ASCII files with (at least) 3 columns, each line containing the X and Y coordinates of the center of the cell and the value of the cell.
The spacing between each cell must be constant and no missing value is supported. Cells with same Y coordinates must be placed on consecutive lines. For a same Y coordinate value, the lines in the dataset must be organized by increasing X values. The value of the Y coordinate can increase or decrease however. The supported column separators are space, comma, semicolon and tabulations.
The driver tries to autodetect an header line and will look for 'x', 'lon' or 'east' names to detect the index of the X column, 'y', 'lat' or 'north' for the Y column and 'z', 'alt' or 'height' for the Z column. If no header is present or one of the column could not be identified in the header, the X, Y and Z columns (in that order) are assumed to be the first 3 columns of each line.
The opening of a big dataset can be slow as the driver must scan the whole file to determine the dataset size and spatial resolution. The driver will autodetect the data type among Byte, Int16, Int32 or Float32.