Set up your workspace for screen
Open units and rulers in preferences
Change units to pixels
Create a new photoshop file 1000px x 400px at 72dpi
Import each of your frames onto a new layer
Open the timeline palette
Choose frame animation
Click the hamburger drop down menu in the far right of the palette
Select make frames from layers
Shift select all the frames and change the frame rate to 0.1
Change the looping of the animation to forever
Open Save for web (legacy) palette
Choose gif