I would say that Gilbert was a silent films actor, pure and simple, whereas Garbo was a talking films actress biding her time, however unknowingly, in silent films.
Maybe you're right about Gilbert primarily being a 'silent films actor', but I probably do disagree about Garbo, in that I think she was good at both. They're just different, in terms of the techniques that are necessitated, and there is much in the movement of silent film acting that proves that the acting is just as much an art as that of talkies--and Lillian Gish is not the only one who proves that, but she's probably the most exemplary. There's Joseph Schildkraut, there's Richard Barthelmess, there's Buster Keaton and all sorts of comedians, there's Valentino (effective and fantastically popular to say the least, whatever you may think of his Thespian virtuosities or the lack thereof.) You can grade talkies more severely than silents if you want to, of course, although I don't see it that way, and there are many who think the greatest films ever made are silents (and some silents are among the greatest, and don't need a more lenient grading--Griffith again, who is incomparable, and Fritz Lang's German silents are generally considered to be far greater than his Hollywood talkies (I'd agree), although some of those are fine, too.)
If you can, see if you can find a vhs or dvd of 'The Kiss'. There's Anders Randolph instead of Lewis Stone again getting cheated on (although I tend to enjoy his recurrence in her films) and the weaving in of the complicated affairs with two other men is quite skillfully handled (those are Conrad Nagel and Lew Ayres, so you have a blue ribbon cast). I believe that, although it's not the last silent film ever made, of course, it was the last MGM silent, or studio silent, or something like that. Dirac or Quiggin will know, and it had to do with fear of her voice not working in talkies (which were already very popular and widespread by then), so they keep the $$$$ signs going all the way until the last sou is collected before they take a chance. And that gives one example of how you may now, in hindsight, see Garbo as a 'talkie actress', but they weren't sure it was going to work till she said 'Gimme a whiskey'.