i've had the same problem on BAMF ROM's for a while. i ended up having to setup the regular Mail app w/ my gmail credentials & it could send just fine from that app.
i'd literally send one email, even just 3 words in the subject line, from the gmail app & watch it in the 'outbox' folder for 5 hours (full sync on, even 'refreshed' to push it). never moved. clearing the app cache/data multiple times didn't resolve it (sadly).
i'd then log into Mail app, place those 3 same words in the subject line & send it & it was instantly sent & 2 minutes later it would be received by the recipient.
don't seem to have that problem on CM7 so i've not looked into it further.
glad to see it wasn't just me that experienced this problem.
will be watching this for any resolution.