First of all, you should be aware that this is not Galuppi's marking. I'm guessing you're reading from this IMSLP file (the passage in question appears on p 17). This is a 1920 edition, edited by Giacomo Benvenuti, and should be viewed as a "collaboration" between Galuppi and Benvenuti (like the difference between "Kenneth Branagh's Hamlet" and Shakespeare's). Benvenuti's introduction makes it clear that he is thinking of performance on the piano rather than harpsichord (and, you know, there are pedal markings); he says he has "rigidly preserved the very few original dynamic markings, but introduced more modern ones, suited to our pianoforte." (Though he maintains an awkward "awareness" of its origins, allowing that "Anyone who wants to play this sonata on the harpsichord knows what to do; and so does anyone who wants to reproduce the harpsichord on the pianoforte." He even suggests that "those who perform these sonatas on the pianoforte use the second pedal throughout and a not-too-strong tone.")1
Here is the same section, in the original source2:
If you compare the surrounding pages with the Benvenuti edition you'll see just how much Benvenuti added. There are hardly any dynamics, no indications to speed up or slow down, or to "play like a violin's G string." If you'd like to play something closer to the original, this source is beautifully clean and readable, though Benvenuti mentioned "copyist's errors." There is also what appears to be a modern "Urtext" edition by Schott, which has hopefully done the scholarship to fix any such errors, but refrains from other editorial invasions: it collects 10 sonatas, but one can also buy pdf downloads of one sonata at a time.
Now, even if sostenuto (come IVa corda di violino) is the capricious addition of a heavy-handed editor, and even if today modern scholarship would never throw such whimsical indications in willy-nilly, it's still an interesting slice of performance practice: It's still worth answering what Benvenuti meant and how one is to do it. Spoiler alert, my answer to the second is "beats the hell out of me." But as to what he meant:
On a stringed instrument, you can play a given pitch on multiple strings. For example, you can play A440 on the violin as an open A string, or in a higher position on D, or a still higher position on G. As you shift up on a string, the tone becomes "darker, thicker," that is, lacking in upper partials. In addition, the G string is the physically thickest, which makes it (in a modern performance style) well suited to a big, beefy, muscular tone. The opening of Ravel's Tzigane, written in 1924, four years after this edition, uses only the G string throughout the violin solo introduction until the piano comes in:
Besides "big 'n' beefy," playing in high positions on the G string is also associated with a more soulful, moving quality (especially in the, I'll say it, somewhat maudlin aesthetic of the turn of the century). I would bet good money that Benvenuti was thinking of what had become known as "Bach's Air on the G String":
(Which is itself an attributive chimera: It is in fact August Wilhemj's 1871 arrangement of a piece by Bach that was originally for orchestra and in a different key.)
I think it's reasonable to interpret a period understanding of Benvenuti's direction as "play with a lyrical, soulful, high-romantic sostenuto akin to a violinist playing 'Bach's Air on the G string.'" What does that actually mean on the piano?
Well, I must confess that I find it baffling any time pianists talk of tone. The only options at your disposal are attack and duration; you can't play this on "different strings" as a violin can. The sostenuto part is clear enough; he wants a very lyrical sustain, perhaps especially where one might otherwise add separation, like in the dotted-16th figure. I have a feeling he might be thinking particularly of the second half of the bar, though, especially with the accent he puts on the middle C. But much of it might be less about tone and more about adopting the kind of rubato that might come from a turn-of-the-century violinist's soulful rendering of this measure. Some sense of this could come from this 1914 wax-cylinder recording of Felix Borowski's Adoration in general, and the 1:00 sul G moment in particular; there is much more rubato overall than in many modern interpretations, and the sul G spot in particular is taken as license to stretch the moment out broadly.
It should go without saying that to fully realize this marking (and the others in this edition) is to embrace a recreation of a 1920s work and its aesthetic. There's nothing wrong with that, and indeed it's perhaps a neglected aspect of performance practice (studying and even recreating moments during the reception history of a piece). But if you want to play in a late-18th-century way, you should ditch all these indications, as well as this edition itself, and play from something cleaner.
- That's my Google-powered translation from the preface at the start of the file. My impression is that coloriti specifically means musical dynamics, but if it means markings in general, he might be explaining how he added even such directions as you're asking about here.
- In case others are curious, the way I found that was: First I found an IMSLP entry for this individual sonata rather than the collection of twelve. This mentions a catalog number of R 50 (apparently this is a system of organizing Galuppi's works, from a 1920s dissertation?), and this line: "Primary Sources: (D-B, Mus.ms. 6997.8)," with a link to RISM, the main database of musical primary sources. The RISM page for this sonata has, at the bottom, "In collection:" and a link to an entry for a holding of 14 sonatas in Berlin, with a handy button saying "See online," where you find a digitized copy.