Basically, you can't run Gaster (or any Ghost) by himself - you need a baseware program first. SSP, the program most commonly used for running Ghosts, is compatible with Windows XP, 7, 8, 10, Vista... probably anything more current than XP. If you're using a Mac or are running Linux though, there are still options for you, but I can't personally tell you how to get them to work. I have heard from people who did get Ghosts running on their Mac using one of these methods though, so it IS possible. But Google Translate will probably be your friend here.
Macs - Ukagaka on Macs, NiseRingo, PlayonMac+SSP (10.6 or above), WineBottler (I think?)
Linux - NINIX, NINIX-AYA
Android - Ukagaka for Android (I have NOT tested this, so...)
If you are using Windows though, just follow these directions and you'll be set in no time! If you already have SSP installed, you can just skip to the end, haha.
If you already have SSP coming into this, then all you have to do is drag and drop his respective balloon zip and nar onto your current ghost, and he'll install. I'm linking here to mirrors on mediafire, but if they don't work, the files are also on the page I made for him I talked about at the beginning.
If you can't get Emily to install the files
What you can do instead is unpack the files manually. If you don't have an unzipping program (I think this is included with Windows now? but), I use 7-zip myself, it's free and pretty versatile. First, extract the z_undertale.zip, so you have a folder called "z_undertale". Then you'll want to go into SSP's balloon directory... it'd be under SSP\Balloon. You should see two folders in there called "ssp" and "emily4" if you're in the right place. Cut and paste the "z_undertale" folder into that folder. Then close and reopen Emily, and check if the Undertale balloon is listed. If so, then you're good!
For Gaster, you'll want to do basically the same thing with the SSP/Ghost folder. First, extract the z_gaster.nar into a folder (a nar is basically a renamed zip file, any zip program should be able to unpack it. 7-zip can do it, for example). You should end up with a z_gaster folder, and inside the folder there should be two subfolders, called "ghost" and "shell". If everything's in place, cut the "z_gaster" folder, and paste it into the ssp\ghost folder, so the ssp\ghost folder should now have both an emily4 folder, and a z_gaster folder. Then close and reopen Emily, and see if Gaster is listed under "change ghost". If so, you're good!
You can stroke his head with your mouse to pet him, or double-click his face to hit him. It'll ask you if you're sure you want to hit him (for the first time, anyway), so you can't do it by accident if you're worried.
Double click his body to open his personal menu. This is the menu with the unique menu options I outlined above.
Right click his body to open the general SSP menu. You can use this to change Ghosts or Balloons.
To check for updates, select it from his functions menu, right-click him and click Check for Updates from the SSP menu, or hit Ctrl-U when he's the focus. He should check for any new stuff and download it automatically. I usually post about new updates in this entry. If you run into any bugs or problems with him, you can leave me a comment there (or here, I suppose) and I'll see if I can fix them.
If you want him to stay on top of your windows, right click him, then go to Options - Preferences, then the Ghost(2) tab in the big menu that pops up. Check "Always Show Foreground" and that should do it.
If you want to make him bigger or smaller, right click him and go to shell scaling, then choose the percentage you like.
If he doesn't seem to be retaining values in his stat screen (like if you pet him five times for example), you can try deleting the aya5_variable.cfg file and letting him make a new one by telling him to reset from his personal menu. You can find this file in the folder where you installed SSP - something like, \SSP\ghost\z_gaster\ghost\master depending on where you put SSP, of course. If the aya5_variable file has been write-protected you might also run into this problem, or if it's in Dropbox. Try moving SSP somewhere else and see if that helps. It's a very lightweight program.
If you want to uninstall SSP, just delete the SSP folder. If you just want to uninstall Gaster, delete the z_gaster folder inside the SSP\ghost folder. You can also turn on the Uninstall option in his personal menu, then select Uninstall from the right click menu.
Gaster's desktop tray icon is a little icon of his SOUL. If you can't find the bell icon while he's running, look for the SOUL icon instead.
Even if you've been treating him well, Gaster may still at times be arrogant, self-absorbed, secretive, brusque or sarcastic. It's just how he is.
If you like Ghosts, you can also check out my Hunter and Smoker Ukagaka/Ghost! They are also pretty complicated and I put a lot of work into them, haha. If you're interested in Ghosts and/or want to look into making one yourself, I also wrote up a big long site about how to do that! It has more information about Ghosts and how they work than I'm getting into here, but a lot of it is technical stuff that's not super relevant to the casual user...
Dr. Gaster, the brothers (c) Undertale: one way to celebrate an anniversary anyway
Okay, so I installed ssp and got Gaster for about 2 hours. Then the program shut down on its own, and now ssp won't reopen. Whenever I click on it, a file labeled "temp" pops up on my desktop. Anyone know how to fix it without having to redownload everything? Edit: I ended up deleting everything; it was crashing my computer. Willing to try it again, though
Okay, I reinstalled everything, got Gaster back, and now I have a different problem. When I close ssp and try to reopen it, it gives me an error box in Japanese that I think has something to do with the balloon/ghost downloads. The box is literally titled "?????" and I can't translate anything in it, but it looks like it wants me to set/reset something. When this happens, the only way I can get Gaster back is to delete and redownload everything, which makes me lose all my progress. Can anyone help with this problem?
how did you get to switch between the bros and him? I still cant get there. Petting seems to be the only thing i can do to progress and I don't want to confess my love to him bc that's weird from my pov lol, really hope it doesn't make me. I refrained from petting for a long time because I knew he preferred that but gave up when it couldnt progress my relationship