@wp_mal hiiii, thank you so much <3
i do it on photopea!! although i think it's largely a matter of learning time, etc, something i do is choose gifs with similar lighting (even if i edit later, it helps a lot to make it look like the same scene), then i try to leave them with the same number of frames, so that when you merge, they stay at the same speed, on deviantart there are several psds for free and then you just download and choose the one you like best and apply it, then both gifs have the same coloring.
about the lettering, i don't like photopea to make, so i create the title in another app (i download the fonts from dafont) and make or mix it the way i think it looks nice, saving it as a png with a transparent background and applying it as a layer for above the gif, but before the psd, then i organize the lighting and blending until i get something that i think is pretty.
i'm not the best teacher, but i hope it was a clear explanation hahah, but if you have any questions, just ask <3