The Save is on your internal SD, it should be CTR-XXXX.sav, Gateway uses the <titleid>.sav.
If this doesn't work, it depends on the games crypt. "CTR Fail" is easy to use with 3DS Explorer + SaveDataFiler.
Beyond that, the fastest way is to use NTR CFW + Savetool on new3DS with Cubic Ninja OR someone with MT Card exports it.
Someone else might need your .sav file and if (if the save depends on it) needed the games header to convert it.