which Function to use?

basically:
=B8&"."&A8&TEXT(C8,"00")&TEXT(D8,"00")&"@gmail.com"
if it is formatted as a table you can use:
=[@Lastname] & "." & [@Firstname] & TEXT([@Month], "00") & TEXT ([@YEAR], "00") & "@gmail.com"

make the e-mail column part of the table so it auto-populates the entire column.
if not a table and you want a single formula for the whole array you can use some of the new text functions or use BYROW with LAMBDA, but I recommend making it a table.