Let's first establish one thing:
Gamp's law is not necessarily violated
Unfortunately, canon is very unclear on what can or cannot be conjured, or indeed when something is being conjured, as opposed to transformed or enlarged.
Hermione does state explicitly what the rules for "food" are:
"It’s impossible to make good food out of nothing! You can Summon it
if you know where it is, you can transform it, you can increase the
quantity if you’ve already got some..."
It's unclear what exactly "food" is here. It must include at least anything that people in Britain normally eat, or it would hardly be a law. There are several conjuration spells that give rise to possible contradictions, but they in turn have possible justifications.
Aguamenti can produce water, presumably drinkable, since Harry tried to produce some for Dumbledore. Thus water may not be food for the purposes of Gamp's laws. However, you "can Summon food if you know where it is." Who is to say that Aguamenti does not simply Summon water from some stationary freshwater source. Or, if this is perhaps implausible, there is always a significant quantity of water present in the air, in the form of vapor. This of course is quite contrary to the situation with beefsteak. It is possible that this charm merely duplicates existing water molecules.
Molly Weasley appears to have produced sauce from her wand. However, it seems most probable that she was either Summoning it or had simply stored it there. In fact, with its explicit reference to both food conjuration and Molly Weasley, this is probably precisely the situation Hermione's quote was meant to explain.
Spells like Avis can produce birds, which are by default edible. However, it is entirely probable that these constructs differ fundamentally from real birds (otherwise magic could produce not just food, but life).
None of these situations, then, present fundamental challenges to Gamp's Laws. Potterwiki insists that sauce can be conjured, but it is notoriously inaccurate. The original text does not necessarily support this interpretation.
Similarly, healing spells do not necessarily conjure anything.
Ron's splinched arm had to be regrown the long way.
When Severus Snape healed Draco Malfoy of injuries sustained from Sectumsempra, it is not at all clear that he was conjuring back Malfoy's blood. On the contrary, he seemed simply to be clotting his blood and joining his skin back together:
The flow of blood seemed
to ease; Snape wiped the residue from Malfoy’s face and repeated
his spell. Now the wounds seemed to be knitting.
Pettigrew's arm was not regenerated by Voldemort; Voldemort had to give him an entirely artifical replacement.
The one big problem, of course, is Skele-Gro. This potion seems to grow back all the bones in Harry's arm over the course of a night, without drawing on his own mineral reserves (which would in any case be insufficient. I see two possibilities. The first, of course, is that bones are insufficiently nutritious to wizards for Gamp's law to apply. This seems unlikely, since bones contain not only minerals, as might be found in any conjured inorganic object, but amino acids such as proline and glycine.
Far more likely, however, is that the a small amount of bone is formed from the components of the Skele-Gro itself, which is then magically increased in quantity. This would not violate Gamp's law, regardless of how much bone was produced.
So conjuring flesh out of nothing would not work. But you might be able to increase the quantity of flesh you already had.
So why did the Trio not multiply chicken meat (or something)
Just because bone can be multiplied, does not mean that flesh can, or that Hermione knows how. She may not even know how to replicate bone. Gamp's laws are not the only limitations to magic. Besides, do they really want to eat nothing but bone broth? Could they survive on that?
Replication may not be infinite. It may well be that magically replicated food can only be increased so much, and loses some fidelity with each increase. After replicating one chicken multiple times, you might end up with a Dementor or something.