My preferred way of capturing video for GIF-making is using
PotPlayer. (It's also my preferred video player.)
PotPlayer's capture menu looks like
this (click here). Here you can capture individual frames or just capture the video.
This (click here) is the video capture menu. These settings work for me for when I later import into Photoshop.
This (click here) is where you click to import a video into Photoshop as frames (mp4 and avi should work). After importing you can add text or whatever which is easiest to do after you've switched into time line view. If you don't you have to work frame by frame.
So to save the GIF you go to File -> Save for Web. Play around with the settings there until your GIF isn't 20MB. You'll probably want to resize the image before trying Save for Web, it might save Photoshop from crashing if the file is too big.
So to recap:
1) Get PotPlayer.
2) Use PotPlayer to either capture frames or video.
3.1) If you have Photoshop you can import the video into a frame animation (or just import the frames into layers and find make frame animation somewhere).
3.2) If you don't have Photoshop you can use something else (which might only work with frames).
4) Save GIF after making sure it's not huge.
5) Upload and use as reaction GIF instead of writing proper posts.