This is one of the big drawbacks of the stock Eris - letting text message thread counts get too high and it takes forever to use messaging or even delete them. Unfortunately, messaging has nothing to do with gmail or anything that you can do on your desktop (that I know of).
There are two apps that you can try to use to delete your texts. I've never had so high a count that I had this issue, but there is an app called Delete Old Messages that can do just what it says - delete messages older than one month (or two months, etc.) Unfortunately, I think it still takes a while, but if you plug in the Eris and kick off the process before you go to bed, perhaps it will finish by the time you wake? Anyway, here it is:
Delete old messages - Android app on AppBrain
There is another app called SMS Backup & Restore. It's main purpose is to backup your messages to your SD card, but it also has a button to delete all of your messages. I'm not sure how fast it is, but remember that this is nuke and pave - they'll all be gone:
SMS Backup & Restore - Android app on AppBrain
Now, I use an app called SMS Backup+ to backup text messages (and MMS) as they are received and sent to a label in my gmail account for archival purposes, so I can use that Delete Old Messages app to delete them as time goes by. I think it's better than storing them all on your phone. They'll still be in your gmail account, so you can always look them up on your phone (and even search for them) from the gmail app.
SMS Backup + - Android app on AppBrain
Best of luck. If neither of these two apps work, I think that your best bet is to factory reset and start from scratch. Note that you could use SMS Backup+ to backup what you have now to gmail (again, give it time), do the factory reset, reinstall your apps (including SMS Backup+), and then use SMS Backup+ to restore the last 100 messages or something, just to have a small archive with you.
