I'm not aware that a gapless-info system exists for WMA. Although I may be wrong, as WMA is not something I follow very closely.If you want to make it into a gapless CD, you can convert the WMA files to wav, trim leading and ending silence (and it won't be all actual zero-sample silence, so you'll need to look at the wavform), and then burn to an audio CD.
On second thought I guess that compression artifacts would prevent the padding from being all zeros. I would be willing to do this manually, since we're only talking about ~20-30 files. I just need a way that doesn't involve transcoding.
gapless encoding is built into the WMA format. I'm not sure if ffmpeg-based players read the data or not. I *believe* it's stored at the end of the file because that's where other codec-related config data is stored.