yourusername+something@gmail.com
Google is unsurprisingly "doing their own thang" there and likely is doing some remapping process. I wouldn't take that example as a correct industry standard - Google have rules unto themselves; seriously. I think some ISPs do similar 'tricks'.
user+domain.com is supposed to be synonymous with user@domain.com, according to cPanel/WHM but this instruction appears to have been removed in later releases. Go figure.
Ammendment
A quick scour of RFCs actually opens the door in what the standards are. The previously mentioned WHM/cPanel "+" delimiter
may have been mentioned to circumvent some now obsoleted email client/server. It stuck in my mind for nearly 20 years, for some reason. :-s
A plus sign may be used in the local part of email and it looks as though CWP might be stripping the wrongly identified illegal character. I don't know without analysing the detail.
My prior comment on email address mapping still stands though.
Bedtime reading:
https://tools.ietf.org/rfc/rfc3696.txt (Section 4.3 is of specific interest)