My Mac recently using RAM like hell?
First of all it's probably a good idea to stop using that memory cleaner app. They aren't necessary these days; if they ever were. The memory systems on modern operating systems are perfectly capable of maintaining themselves. The way these memory cleaner application work is by claiming a huge chunk of RAM, thereby forcing everything not critical out of RAM, and then releasing the RAM they've requested. Your swapping something that has been fine tuned over years by a lot of very smart people, with some guy wielding a mallet.

This could actually be the reason you are seeing your free RAM drop to zero. It could be the memory cleaner app itself.

The idea that you need to have a large amount of RAM sitting "free" is just wrong. When you close applications the RAM isn't necessarily immediately freed. All modern operating systems keep recently used files cached in RAM so that if you (or the system) decide to reopen something recently closed the files open quicker.

If you do have a performance problem with Gmail in Chrome the problem is likely elsewhere. I highly doubt it has anything to do with the amount of free RAM on your Mac.