You see them pretty often in major U.S. metropolitan areas, most notably those with a large tourist base and mild weather - New Orleans and Austin, TX, for example. Dreadlocks or mohawks and a plethora of piercings and the occasional facial tattoo. Their clothes are dirty, and they travel in groups, with all their belongings with them. Quite often, there will be a mutt or two, wearing a bandana and often better behaved than the rabble he belongs to. With hastily made cardboard signs, they set about panhandling for beer and food money.
These are the gutter punks.
Often homeless by choice, they tend to travel about the country, hopping freight trains from city to city, traveling south for the winter and northward for the summer. It's a lifestyle and a network created on the go, with groups bunking up in new squats as they arrive in new towns. New friendships are forged that may last a day or a lifetime.
Also called crusties and associated with the crust punk sound, the movement has risen in numbers since the '90s. Whereas the idea of a punk squat began much earlier in the UK and throughout the U.S., the idea of the transient gutter punk movement is much more recent. It's based around the life of the hobos of old, although hobos rarely had dreads or mohawks, to my knowledge, nor did they have a musical movement surrounding them.
In addition to the music typically known as "crust punk", another sort of musical scene has become attached to the gutter punk movement. Much more folksy in nature, it shares it's sounds with roots, Americana and Gypsy Punk, due largely to the fact that much of it is a transient musical movement performed on the roads by the gutter punks themselves, on acoustic instruments that travel with them as well.
In addition to panhandling, most gutter punks sustain themselves through dumpster diving. A movement also known as Freeganism, many established as well as transient gutter punks and squatters maintain this lifestyle, not only as a means for cheap food, but as a statement against the conspicuous consumption of consumer culture, maintaining that they are (often rightly so) doing their part to reduce consumer waste as well as minimize the amount of resources they use.
Of all aspects of gutter punk culture, Freegansim is the most organized, with groups discussing strategies, community and cooperation through resources like the online destination Freegan.info. Essentially, freeganism has more advocates that also maintain a certain permanent residence, which includes access to the internet and postal contact. This allows them to help maintain a broader sense of community.
One of the most famous self-proclaimed gutter punks is former Crimpshrine frontman Jeff Ott. In his book, My World: Ramblings of an Aging Gutter Punk (available for sale here), he has compiled excerpts from hs zine of the same name, consisting of his observations and recounts his life as a homeless punk, as well as dealing with drug addiction and his subsequent recovery.
Some gutter punks maintain the lifestyle for a limited time, before deciding to settle down and integrate back into mainstream life. Some do it for the entirety of their lives - which can and do end prematurely due to dangers intrinsic with the lifestyle (A fire in a New Orleans squat in 2010 claimed the lives of 10, aged 17-29, as described in this story). But as a movement, gutter punks are a solid, if disorganized by definition, piece of the punk subculture puzzle.