Use the File > Open command and choose Text from the File of type list. Next, specify additional text file options:

Cell separator	A single character used to separate cells in one line.
Fixed cell widths	A list of fixed cell width values. For example: 10,30,15,40 If not all widths are specified, the remaining cells are assumed to contain 1024 characters.
Quoting symbol	A symbol used to delimit fields containing separator characters. Cells containing quoting symbols, separators or new line characters must be delimited by quoting symbols. The inner quoting symbols must be doubled. If you need to open a text file that doesn't conform to this standard, consider either de-selecting this option or specifying some unique quoting symbol that doesn't occur within that file.
Escaping symbol	A symbol used to mark the occurrence of a single byte value specified as a two-digit hexadecimal number with the preceding x letter. For example: \x66 \xA0\x45 Escaping symbols not representing the above sequences must be doubled. Escaping can be selected when opening a text file. When saving GS-Calc always uses the more common quoting method along with utf8 encoding. By default escaping is turned off - choose it only if you're certain that a given file uses this form of encoding and uses the specified escaping symbol.
Note: If a given text file is to be opened entirely as textual data without any text-to-numbers or text-to-formulas conversion/parsing, you need to clear the below parsing options. After opening, all worksheet columns for that file will automatically be formatted using the "Text" style and any entered data will consistently be treated as text. To delete the "Text" style for a given column and to turn on parsing numbers and formulas, select that column and either press Del, then "Formatting" or choose any other numeric style on the formatting toolbar.
Formula conversion	Specifies whether loaded text strings should be evaluated and converted to formulas. Leaving the default No Conversion option results in treating all cells as text labels.
Convert text to numbers	Specifies whether loaded text strings should be evaluated and converted to numbers. Only unformatted numbers can be converted. The decimal point is determined by the Settings > Locales menu options and by the current system regional settings.
Parse formatted numbers	Specifies whether text strings representing not only plain numbers but also formatted numbers should be converted to numbers. The decimal point is determined by the Settings > Locales menu options and by the current system regional settings.
Convert date/time strings to date serial numbers	Specifies whether text strings representing dates and/or times should be converted to date serial numbers.
Convert date/time strings to date serial numbers	Specifies whether cells containing numbers (obtained after the text) should retain the corresponding data styles.
Encoding	Specifies the text encoding for a given text file: UTF-8, ANSI 8-bit, ISO/OEM.
Maximum columns in sheet	By default, if a given text file has more than 16K columns, GS-Calc warns about discarding the data that don't fit the sheet dimentions. If you check this option, multiple sheets will be created to load the remaining columns above 16K. You can specify any other splitting limit from 1 to 16384 columns. The opened text file can have up to 1 million columns that will be splitted in 16384-column sheets. If multiple worksheets are created by GS-Base, trying to save back such a set of split sheets as a text file will result in prompting for a new ZIP (zip64) file archive file name instead of overwriting the original source text file. To load such ZIP text archive later for editing, simply choose the "Text | Text Archive (*.txt, *.csv, *tsv, *.zip)" filter in the "Open File" dialog box.
Maximum rows in sheet	The same as the above but for rows. You can split text files into multiple sheets with any number of rows from 1 to 32M. Note: for very large text files that can't be loaded to RAM, you can use GS-Base to process files of any size and filter specific sets of rows for GS-Calc.

Select the Settings > Options > General > Auto-fit column widths in imported files option.
(See Settings)

Use the File > Save command and choose Text from the File of type list. Next, specify the same additional text file options as above.

By default, if you choose the "Text | Text Archive (*.txt, *.csv, *tsv, *.zip)" filter in the "Save File" dialog box to save a given *.gsc workbook as text, only the current sheet is saved as a standalone text file.
If you want to save all sheets at once, you need to specify a file name with the *.zip extension. This will tell GS-Base to save all created text files to one ZIP64 text archive (along with a short XML file that specifies used text parameters so you won't have to re-enter them when opening that ZIP64 file in the future).
When saving ZIP text archives the options to split sheets based on the number of rows is inactive.