Nothing complicated... I was happy to find it just works: open the GIF in Photoshop, scale it 2x (or whatever) with Nearest Neighbor scaling, then Save For Web as a GIF. Animation timing is preserved!
Very handy as I finish my Outside Parties trailer for the next Tiny Yellow Machine Community Direct!
Good to know!
I have a right click menu item installed on macOS to quickly 2x a GIF. I think behind the scenes it uses command line image magick (but I'd have to confirm).
edit: in fact I use gifsicle
Aseprite also handles .gif quite nicely, and it's made primarily for pixelart, so nearest-neighbour is already selected by default when scaling.
Just select 'Sprite' > ' Sprite Size...' from the menu-bar, then:
I used it previously to cut off excess .gif frames to get a file beneath the 4MB limit used here as well.
(It should also be available on steam. My friends know I'm doing a game-jam if they can see me 'playing' aseprite... 
)
Now if you export video/audio from Mirror instead, it has a 2x option built in. (But sometimes a performance penalty.)