Multiple signatures in Google Gmail on my Mac system?

You’ve picked up on one of the few things I also dislike about Gmail, it’s poor support of multiple signatures. Perhaps everyone on the Gmail development team has a single signature that they use for work and personal mail so it’s not an issue for them? In any case, lots of us have more than one hat we wear, whether it’s multiple professional gigs or whether it’s the separation of business and personal email communications, and getting them to work in Gmail is a pain.
Before the switch to the new pop-up compose window, I simply used a signature that contained both my biz and personal sigs and just spent the extra few seconds chopping out the unneeded info before clicking “Send”. But that was clunky, and since the signature is now part of what’s hidden behind the “…” button in the compose window, it’s way too easy to forget and send the entire signature mess, not a good outcome at all.
Gmail also has “Gmail Labs” experimental plug-ins for Gmail (you can find them all in the “Labs” option along the top of “Settings”) but none of them — surprisingly — help out with managing multiple signatures either.
Instead, what I’ve done and what you might want to explore is to use the built-in keyboard shortcut feature in Mac OS X to create succinct abbreviations that auto-expand to your full signature, abbreviations like “-sig1-‘and “-sig2-“. There’s a lot more you can do with these keyboard shortcuts, but let’s just consider this particular use case.
First step is to go into Gmail’s Settings and disable your existing signature. That can be found on the “General” settings, scroll down to find something like this:

As you can see, a click on the button adjacent to “No signature” disables it without you having to delete your existing signature (just in case you want to switch back).
Now from the Apple menu, choose “System Preferences…”

The keyboard shortcuts mapping feature can be found in an area that most people have never explored: “Language & Text”:

Click on the “Text” tab and there are a bunch of default expansions, including replacing “(c)” with the copyright symbol. I also add “teh” –> “the” to help with my occasionally sloppy fingers:

To add a new expansion that’s going to be one of your email signatures, click on the “+” in the lower left corner of the window, then type in the abbreviation you want to map, followed by the expansion text:

A handy tip: make sure you don’t map a sequence of characters or digits that you might otherwise type, or it will drive you utterly insane. For example, you can drive a colleague batty by mapping “a” to “e” or “th” to “ht”. Every time they type things will instantly transform without any explanation or obvious reason. Sneaky!
I’ve set up five different signatures because, well, because I can:

Unfortunately when the text passage is longer than the tiny text window, there’s no pop-up, no way to see exactly what it looks like without actually using the shortcut, so my recommendation is to create the signatures in a simple editor like TextEdit, then copy and paste them into the Language & Text expansion box.
With that set up, it’s incredibly easy to use and works across all apps (even Apple Mail!). To demonstrate, I’m in the Gmail message composition window and I’ve typed in all but the very last character of my signature expansion:

Instantly, as I push the “-” to finish the signature sequence, it’s replaced with all the text I’ve specified in the preference:

It’s not the most elegant solution, perhaps, but quite functional, and with a tiny bit of practice, you’ll be picking the right signature in no time.
Note that there are a lot of keyboard shortcut expansion tools available for the Mac system, many of which are considerably more sophisticated than this built-in feature in Mac OS X. If this is a bit hard for you to wrap your head around, I suggest you check some of those out!