Let’s assume you have your emails entered in column B, then you can enter a formula in column C, which only selectively shows those emails that do not contain “@gmail.com”. Afterwards you can sort the table by column C (descending order), which lists all non-gmail rows at the top. This allows you to select the bottom rows and delete them in one step.

The formula in the cells of column C is:

When you fill in the formula for one cell, you have to copy the content down into all rows of your large table. The easiest way is to select all the relevant cells of column C, and then click on “Autofill Down” in the menu ‘Table > Autofill Cells > Autofill Down’ - this will copy the formula into all rows. (Or you drag the cell with the formula - yellow handle - downwards across all your rows)

Here are some screenshots:

The you can sort the entire table by the column C by selecting ‘C’ and ‘Sort Descending’

It should result in a sorted table:

You can then select all the ‘bottom’ rows and delete them if necessary

The process still requires some manual steps, especially selecting many of your 20K rows, but that should be easy with scrolling and shift-clicking. Of course, you need to adjust the formula to the proper columns used in your table.