Reddit mentions of Adobe Photoshop CC Classroom in a Book (2015 release)

As everyone else has mentioned, you do seem to have a lot to learn. There are a ton of good books, youtube tutorials, and other communities that offer educational sorts of resources. Especially if you want to learn very specific things, it's very easy to find instruction.

As for the 30 day trial, tbh that's not enough to "learn" Photoshop. Most people who use it professionally have done so for years, decades even. The good news is: for what you want to do, you don't have to learn all of Photoshop. But it'll take longer than 30 days. I highly recommend checking out the different licensing packages and payment options available for Photoshop CC, as well as the entire CC suite of applications. It looks like the student pricing is currently $20/month which is an absolute STEAL if you plan on using anything more than Photoshop. Illustrator is going to be better for your graphic overlays (although Photoshop is more than capable there as well, just kinda feels like you're constantly in workaround mode - for me at least). After Effects and Premier are awesome for video work. Audition for audio editing. Etc. Etc. And the student pricing does not affect licensing as far as I know - everything I've read says your outputs are your own.

As for a brief list of what you'd need to learn in Photoshop, the graphic overlays and avatars you're talking about use essentially the same skill set (unless you plan on painting your avatars with a graphics tablet). You need to learn the layers window backwards and forwards: stuff like adjustment layers, blending modes, opacity/fill, layer blending styles, masking, text layers. You'll also probably want to learn the pen tool, shape/custom shape tool, and selection methods. Unless you're painting your own images, you don't need to worry about the brushes so much right away but it will help you in the long run to be well-versed in creating/editing your own brushes as well as how to get custom brush packs off the interwebs.

Creating GIFs is a slightly different animal. You still need to know the stuff above, but not as in-depth necessarily. But you should understand the animation options that Photoshop has and how to work within that portion of the application. It's not difficult really. Mostly it involves importing a video segment, making whatever edits you need, then exporting it. But the making edits part means text, layers, selections, masks, and so on, so it's nice to have a base understanding of those things. Creating your own GIFs, as in from your own drawings/graphics flipbook-style is also not particularly difficult. Just lots of layers. But you'd need to know the digital painting stuff (brushes, various editing tools, filters, and so forth) for that.

These are very small slices of what Photoshop can do, but they do dip into the skill sets of other tasks like 3D art and photo editing. I recommend getting a really good Photoshop book. There are so many and I haven't read most of them. I see a lot of people toting this while learning Photoshop. I used this series' book for a different application but it was REALLY good so I imagine their Ps book would be no different. Personally, I think when I was learning game art the book we had to get was this one, and it was decent. I didn't get as much use out of it because I knew the photo editing side of things already from previous experience, but my own personal experience shouldn't detract from what appeared to be a very knowledgeable and well-laid-out book.