The Apple Photos app keeps all photos, thumbnails, application settings and other image/video meta-data in it's own package file located at ~/Pictures/Photos Library.photoslibrary. As such, you won't find the original images or videos in a normal Spotlight search.
Native OSX applications can present themselves to Photos as a compatible extension. You can see these when right-clicking on a photo or video and selecting share.
Exporting the video from Photos
As gmail is not a native application with this capability (and a quick google search suggested no alternatives), you'll need to export the video and attach it manually.
Select the video in Photos and choose File -> Export
You can choose to export the video unmodified. However, as you'll be attaching the video to an email, you may want to choose Export 1 Video and select a smaller Movie Quality so the video will fit within gmail's attachment size limit of 25mb.