I see Markiplier playing a lot of games that are made with Garry's Mod. So, I want to know if it is worth the $9.99 in the Steam Store? 

It depends.
I was NOT satisfied with my purchase.
In order to BE satisfied, you will need:
A ton of textures from other games (most you need to purchase)
Be ready to download mods.
Have a ton of friends to communicate and play with.
I recommend going on a server if you get it, building is unnecessarily hard.

thing1 : there are free texture packs to correct the missing textures but it is not really all that fun without friends i like mine because almost every one on my friends list has it so yea it is fun in short burst for me gets old quick 

thing1 : I got it from Laian for 15k Viz while it was on sale but I think it is worth it. There are a ton of fun things to do granted you have the space. Yes, there are a lot of things that require textures but you can get free alternatives most of the time and a lot of the mods don't require the external textures, at least not the ones I use. It is a great game to play with friends and if you are worried about not having anyone to play any gamemodes with then just ask around on vizzed , I'm sure you can find some people to play with. It is a great experience and I would recommend you get the game, though you might want to make sure you have a lot of hard drive space if you really want to get the full enjoyment of the game.

thing1 : If my copy of GMod was physical, it would be locked away in my basement covered in Dust and dead cells like a Bezoar. Save your money for Half Life 3 or some sort of Black Mesa: The Pre-Gordon days.

Well, I definitely think that it's worth it. The game has been more popular than ever, with YouTubers like Vanoss Gaming uploading videos of playing Garry's Mod with friends. In addition to that, the game provides a sandbox experience like no other. There are some elements of the game that you won't even find in a different sandbox game like Minecraft. Overall, you are going to have a blast with friends, whether it be building or playing on a map.

It really depends. You have to be patient and creative, the two things I am anything but. I bought it and I feel like it probably wasn't worth it for me. The whole point of Garry's Mod is to do whatever you want, but I just can't come up with any ideas. You'll also want to have a lot of Source games if you want to make stuff or have more objects. If you join an online server they download a ton of items, so be prepared to wait for hundreds of megabytes to be downloaded for a single server. You'll also want a large storage drive if you want to play (get a solid state if it's not too much for faster downloads).

thing1 : Ignore all above responses, trust me on this. Source: me and my 500+ game steam library.

First thing; Garry's mod always goes on sale, during a sale it will be reduced to around $2.50 / £1.40. Chances are it will go on sale between now and the Summer Sale and if not it will be on sale during the aforementioned sale.

Secondly; I've checked your steam library and you already own all the games required for textures. You'd need them installed on your machine though; main ones you need are cs:source and Half-Life 2. (ep 2 also recommended.)

Finally; Gmod offers a lot of outlets, you can do creative stuff i.e. sandbox or you can join community based servers for things like ttt, prophunt and rp. As someone with over 500+ hrs on it I can safely say it gives 80% of players something they enjoy, I highly recommend buying if you have friends that play.

ps. you really dont need to have a lot of system memory for server stuff unless you're going on s***ty servers with bloated crap on it. If you want to know some good servers send me a message on steam (I'll look offline but I'll see it eventually)

The gmod is the best cheap game there in steam, has achievements, trading cards, workshop and a great community.
As iN008 says you should wait for this on discount not only reduce the price in holidays.
It is one of my favorite games (played 3,260h) not only because I love halflife and counterstrike (which also I have all) also because you can reproduce and fuse other games and create a completely "new game" and so began to birth the "gamemodes" now there are hundreds of modes and within a mode you can find different ways to play...

I like combine different mods and rebuild, move around, touch up, mess up and destroy!!
You can make scenes (photos, posters, art ...) or you can create movies and series...
I created my own server, and can put what I want and try it with friends... tons of fun...
The "gamemodes" are completely different "Zombies" "Naval battles" "Role play" "Golf" and a large ETC...

Conclusion: GarrysMod is a essential-own game and everyone should have it! 9/10
Steam Rank: "Extremely positive" The 96% of 138,259 users analysis are positive about this game!

...you need it! hahaha

GMod is only really worth it if you have enough people to play it with and do it online. I saw Mark playing it and got interested in it from him and then I played it for a while though I have the regular version and if you get the regular version, some maps won't work. The game is a lot of fun and should last for way more than just a couple of rounds, and since GMod comes with so many different mods especially Prop Hunt and Trouble in Terrorist Town, it's defiently worth it because it's basically multiple games in one.