The late 1400's Florence is actually an area of interest for me. I participate in medieval/renaissance re-creation events, and my persona is right within the realm you're talking here.
The gamurra was also known as "gamurrino, camora, zimarra, camurra, zottana, gonella or zupa", so you can always take your pick there, but camicia, sharing Latin roots with the French "chemise", I always thought was more on the "similar enough to figure out without too much trouble" side. However, you could also go with "camisia, chemise, smock and undergown".
Depending on the season, you could use a "cotta", which was a lighter, thinner version of the gamurra.
The giornea really does have no real equivalent in my experience, as it was just that temporary fad within a span of maybe fifteen years. Slashing to show the camicia or undershirt (for men) stuck around a lot longer. Predominantly it was a tabard-style overdress, which could be open on the sides and the front, or a combination of either. It could have closures at the front opening, or merely hang loose, and was often cinched with a "cinture" or girdle, in this case generally a "belt" of fine fabric, decorated with gold, pearls, beading, or even gold chain, so not the kind of girdle we think of today. From the cinture would be hung items of convenience, like fans, perfume holders (because not everyone liked to bathe), small bags, or decorative tassels.
Neat side note: They had pockets. Well. Sort of pockets. Well . . . more like a purse accessible through a slit in the gamurra. The saccoccia (like sack, or satchel, yes?). Some were sewn to a gamurra, others were attached at the stays, or belt. Those more of the working class would often wear it outside the gamurra, rather than beneath.
So . . . now that I've gotten completely off topic, I'll say that I find myself defaulting to "chemise", "underdress", and "overdress" when talking to people who aren't familiar with the Italian terms, but if I was writing an Italian-esque setting, I'd prefer to use the actual Italian words. Even if you come up with -different- words, you're still dealing with teaching people through context what it is you're talking about. It's like dropping words from any language, real or invented. It can be done. I've seen people do it, even.