Regards to all.
I suggest using Font Book, selectable by choosing "Manage Fonts" to check out what a font contains (if you already have it). Choose Preview / Show Repertoire, and you can scroll through everything your font contains. The ligatures are instantly recognisable, being double or triple characters.
You won't have to choose these as characters. It's simply that if they are present, they are available to the kerning rules Pages applies, depending on your selection as to whether to use standard or all. Should you have chosen "all" and only standard are available, the result will be no different than had you chosen only "standard."
Having seen what your font provides - and that may be beyond just ligatures - the typography panel will help you exploit them. You can also use it first to check whether Preview Repertoire is going to interest you. Basic typographical features will shown, but not exampled, in the panel this brings up.
But if you want more detail from it, you will need to have entered the font you want to look up in pages - and for details as to alternates, etc., you will need to have entered some text. The alternates available for that will then be displayed for every character in your entry. Don't make it too long, since this is really for detailed work. And what you'll be shown is pretty schematic, so it helps to have seen the Font Book preview first.
Generally speaking, Pages will use the resources contained in a particular font. But if you've bought a supplementary set, such as small caps for Baskerville, say, you'll need to switch to that in order to use them, or Pages will simply scale them from those in your basic font.
As to your Garamond Pro, check its repertoire as I've indicated, and then compare it with your standard Garamond. If the pro set has the lot, you may not need the basic font - but if it only contains additional resources, you will need to continue with both, and select the Garamond Pro when you need to use what it has. Chances are though, that an Open Type Pro set may contain the lot. It would be nice to see that as standard; and day will come when we do.
As to your Gentium, if you have it, the above will answer your question.
Otherwise a search on Fonts.com, or other type suppliers, might answer that question for you: as with other fonts you may be considering, but do not yet have to inspect.
Topnotch typography is what Pages has foremost to offer us.
I'd bet many computer users can spot a document printed from word, or Appleworks & etc. on the instant. They conform to what the programs offer.
But with Pages, if you go your own way with it, you can look like you've been to the printers and paid for a professional job.
My heartiest cheers for this topic.
