this is a fault of the GMail app, as opposed to the HTC Mail app.
GMail is a bit finnicky and won't necessarily download the attachment(s) to your phone from your email--it will only try to 'preview' it, and Videos are a PITA to Preview. so, use the HTC Mail app.
(similarly, this is also the solution for people looking to download a .pdf file through their GMail app)
here's the solution for the 3gp file:
open the email in your MAIL app (HTC app). tap the 'attachment' portion of the email, you'll see it spinning for a moment, then turn into a Paperclip icon. (if it keep spinning, try opening the email while on WiFi connection)
when you've done this, you've downloaded the attachment to your phone\sd card\downloads folder
now, do 1 of 2 things
1) app tray>videos>watch the video
2) use a file explorer to navigate to the phone\sd card\downloads folder and tap the video & choose which Video player you want to use to view the video.
for the m4v file:
sadly, the M4V isn't supported on Android. perhaps an alternate video player from the Market supports it (like vPlayer)--but not the native player itself: if it is a true M4V (apple/iphone/ipod) video file--as something usually laidened with DRM--if it were purchased through the iTunes store then it won't be playable on anything but an Apple device.
