You can always download the drivers
from Gateway, and I'll give the usual link that's always posted on here, but you can download your own .iso image for the version and platform of Windows 7 you have a license key for. It's a plain disc, not a recovery.
You wouldn't have to do anything for drive letters, if you did a clean install. Just remove the HDD, install the SSD, and then install Wndows 7. Once it is up and running, add the HDD back in. It would likely be E, if you have an optical disc drive, but you can always go into Disk Management and swap them...to make the HDD D and the ODD E. Official Windows 7 SP1 ISO from Digital River « My Digital Life