For people looking to transfer their photos from Gallery to Flickr, I have spent $200 for you.
A nice fellow I met on Flickr, Myrmi, made a nice little Gallery2Flickr script for me. And I have just recently finished uploading all my pictures to Flickr.
So here, I share the script with you.

If it does what you wanted, please consider making a small donation. Cheers.

Myrmi's great Gallery2Flickr script:
Upload it into the "albums" folder (the one with all your albums) and edit the variables at the top of go.php.
You will of course need a Flickr account (preferably a pro account if you plan on transfering more than 3 albums)
You will need an API key, which you can apply for here. Once you've edited go.php, just browse to the go.php file in your browser and everything will work.
The script:
- requires FTP access
- doesn't require access to command line.
- lists all your albums and you can upload them one by one.
- will not reupload pics that were already uploaded in case of crash or else.
- only uploads supported file formats on Flickr
- puts all your pics in Flickr Sets named after the albums in Gallery.
- preserve all captions
- saves your keywords in Gallery as tags in Flickr
- saves your pictures' comments in Gallery into your captions in Flickr
- preserve chronological order.
Works with Gallery 1.x I guess.
The script uses the PHPflickr library for this, which is included in the download. I also donated part of the $200 to Dan.

NOTE: This script is given as is. I am not responsible is smth goes wrong. It transfered my 3500+ pics without a hitch so it has all the chances to work for you too. I cannot trouble shoot nor modify this script for you so please do not make any requests in the comments for that.