Gregory and Sampson are flat characters. They appear for only a short scene at the very beginning of Romeo and Juliet only to spout off their "locker room" type humor.
Shakespeare's purpose in placing Gregory and Sampson at the very beginning of the play is to grab the attention of the audience, the groundlings in particular. Almost every one of Shakespeare's plays begins with one (or more) of three elements: references to sex, violence, or witchcraft. In Romeo and Juliet, we see two of these elements: sex and violence. Gregory and Sampson are not round characters because Shakespeare doesn't want them to be. They are in the play to do their job and move on.
First and foremost, whether a character is "flat" or "rounded" is simply a critical opinion, and nothing absolute in the text of the play. Some people might argue that King Lear is a flat character - simply a ranting, raving old man - where most critics would disagree. It's up to you - and how you read the play.
So whether Gregory and Sampson are "rounded" or "flat" depends whether (and this image, of course, refers to how three-dimensional they are - whether they, to coin another phrase, leap off the page) you think there are many facets to Shakespeare's characterisation of them.
All they talk about, really - before the violence breaks out - is women, and how they are going to have sex with Capulet's maidservants (in quite a violent way). Sampson, in particular, named after the Biblical strongman, puts huge stress on his own physical prowess. Are they flat characters? Are they fully or partly rounded?
Or - and I think this is the best interpretation - are they deliberately shallow presentations of shallow, street thugs without a great deal of depth to them anyway - just like in life?