Not Gonna Lie
Indicating you are admitting something that may be considered strange
Could be deemed the equivalent of 'just saying' or 'no offence' in the sense that it makes your statement more valid/less offensive

Noun: a friend who is usually only called upon by a friend when a more primary/prominent friend is unavailable.

Taken from the term “second string” in an athletic competition situation. In football, if the star quarterback gets injured during a play, the second string is called off the bench to replace him/her. A “Second String Friend” is essentially benched until needed, if ever.

Synonyms: Plan B, Second Choice, Secondary Friend, Benched Until Needed

not gonna lie. A pre-sentence or post-sentence addition that assures the recipient that you are indeed not lying.

Not Gonna Lie

Used to indicate that you are not deviating from the truth.

NGL - Not gonna lie
Indicated you are going to tell the truth