Imagemagick is hard to use and the documentation/tutorials available are not intuitive. Here is a concise explanation of what needs to be done to edit a gif to add labels.
convert foo.gif \ # Specify the original gif
-gravity center -pointsize 30 -font ComicSans \ # Set font position and styling
-duplicate 1,0-20 \ # Clone frames and append to the end (clone frame once)
'(' -clone 21-40 -fill white -annotate -0-150 'Hello' -set delay 6 ')' \ # Clone frames, add 'Hello', and slow down gif delay for only these frames
-duplicate 1,41-89 \ # Clone frames and append to the end (clone frame once)