Ffmpeg can convert video files to gifs and vise versa I use this one but there are others
https://play.google.com/store/apps/details?id=com.silentlexx.ffmpeggui
Transcoding video is hard work for a phone. It might take half an hour to convert one that long and your phone will probably get hot. But it should work.
There a hundreds of video editing apps , I've yet to find one with frame by frame accuracy. Text always seems to appear at the nearest 1/4 second. Just browse around and find one with a gui you like.