These strings are a favorite of jazz bassist Ron Carter, and are pretty popular for that and other reasons. They are metal rope core strings, surrounded by a black nylon wrap, for a smooth feel.
According to LaBella, they were originally designed to emulate gut (see their description below, in italics), but with a bit more of the feel and sustain characteristics of steel strings. As a result, many folks who try these strings expect the timbre to have a thumpier, more gut-leaning sort of vibe – with a blunter attack, quick-ish decay, and a shorter sustain characteristic. these strings actually have a fairly well-defined attack – almost like a steel wound string – and a somewhat more steel-string sort of decay and sustain, though they do have a nice “thump.” They’re just slightly “dampered” to prevent brashness, if that makes sense.
The bottom end is good, and the steel core gives them a strong fundamental. So, my subjective opinion is that I don't think that they sound all that much like gut - their pitch is clearer and sustain longer. Rather, I feel that they sound more like a slightly "muted" steel string.
So, it depends what you're looking for; they are a good string, and COULD be (and are) perfect for many players – but a lot of folks go into them expecting a dark, thumpy string (which is a reasonable expectation with the nylon windings), and they really don’t quite match that description. Just thought I'd let you know.
Like most strings with a nylon or perlon outer winding, they're not a great choice for arco (bowing) as the bow has trouble "gripping" the material. You can work with alternate rosin choices, and even "scuff up" the outer surface a bit with fine steel wool, to create some purchase for the bow, but coupled with the stiffness of these particular strings, bowability is definitely not their strong suit.
Labella does not list gauges, but I have taken the time to measure all the strings using a digital caliper and present the following unofficial approximate measurements for your reference:
Over 30 years ago, Ron Carter came to La Bella Strings with a vision: a jazz string that captured the rich tone of gut and sustain of steel, but without the headaches of gut and overly metallic sound of chrome. Months of R&D alongside Ron Carter and Attila Zoller resulted in the formation behind the 7710N Black Nylon Tape Wound Double Bass set. Wound with a black nylon tape outer-wrapping, this unique string design is ideal for jazz pizzicato, particularly solos. The result is a bright attack with a warm tone, creating a set truly unparalleled by any other double bass set on the market.