dont want to derail this discussion from the main focus, but i will say i dont have any issue sending large files as long as they are under the gmail file size limit, so i dont have the issues you are talking about today.
nowadays i use cloud storage services to send files.
in your accounts and sync tab, verify if there are any error messages/sync error messages.
go to yoru google dashboard:
if you are using chrome sync, clear the data there and resync.
verify you are on a reliable data connection with no data restrictions (i am sure you are good here, just wanted to mention)
i ASSume you are not rooted?
worst case, you can factory reset and/or remove the gmail account and re-set up.
looking for software gremlins over the internet is difficult to diagnose.