@Giovanny_Giraldo So the Loop is a piece that takes an array (collection) of items and perform an operation for each of these items.
If you got all the rows by the Find Rows action of Google Sheets, then you can add a Loop like this and select the Find Rows step as an input for it:
Now, add the Gmail step inside the Loop, and start filling its settings by using the Item data item like this:
Complete the pre-filled value with .values.COLUMN_LETTER like .values.B if the email address is under the second column in this case. Like this:
Follow this approach for the rest of your flow. I hope this helps!