gmLoot

At a glance, I don't see any global leakage, so that's good. The only issues I see are minor, and fixing them would mainly be educational.

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You should never specify both OnTooltipShow and OnEnter/OnLeave. When possible, just use OnTooltipShow, and let the display handle showing, positioning, and hiding the tooltip. The only reason to use OnEnter/OnLeave is if you're using a custom tooltip object (like LibQTip) or want to do something other than show a tooltip when the user mouses over your plugin. In your case, just use OnTooltipShow.

Also, your OnTooltipShow method doesn't get your data object as its first argument, but does get a reference to the tooltip object being used, so this:
should be rewritten like this:
and then inside that function, you should refer to "tooltip" instead of "GameTooltip". In the vast majority of cases, the display addon will just use the GameTooltip, but it doesn't have to, and in cases where it doesn't, your code won't work.

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You're currently doing this in both your OnClick and OnEnter methods:
Don't do this, in either place (though you should be getting rid of OnEnter anyway). You should leave it up to the user's chosen display whether they want to show the tooltip and/or respond to clicks in combat. The only good reason to handle yourself is if your plugin does things that can't be done in combat. Personally, I find it bad enough that my display addon (Bazooka) defaults to "no tooltips in combat" for newly installed plugins, and then I try to mouse over it in combat, and get nothing. If I didn't want to see the tooltip, I wouldn't put my mouse there!

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"elseif" is your friend!
Change all those "end ... if"s to "elseif"s. Currently you're running all 3 checks, even after you've found the only one that will pass.

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You don't need to (and shouldn't) create 3 separate frames to handle events. One frame can handle multiple events:
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When you initialize a saved variable in the main chunk (eg. right away when WoW loads your file, not waiting for any events) it will always be nil so there's no purpose in checking whether it's anything else.
GMLOOT will always be nil here, so it will always get set to {}. Saved variables aren't loaded until ADDON_LOADED fires for your addon, so it's safe to set the initial value in the main chunk with just:
The next line has an additional problem:
Not only will GMLOOT["COINS"] always be nil here, but if this line ran after the actual saved variable had been loaded, and the user had turned off coin looting, since that preference would be stored as the boolean false, this line would overwrite their preference with true, turning coin looting back on.

You should replace these three lines with this:
This way, GMLOOT is defined as that (default) table when your file is read, then gets overwritten by the real (saved) table just before ADDON_LOADED fires.

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You could use boolean values more effectively. For example, you can simplify things like this:
to this:
And instead of this:
Do this:
You can also work with CVars as booleans. Instead of doing things like this:
you can do this:
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Don't print stuff every time the player logs in. Give them credit for being able to remember that they installed your addon and configured it to their liking. When it's just your addon spamming, it's maybe not so bad, but imagine that every addon you use prints even just one line of text when you log in. That's a ton of spam, and not only is annoying, but also ends up hiding things that actually matter, like your guild's message of the day.