At various times over his career he painted scenes from the epic Finnish poem, the Kalevala. Some were easel paintings, some were murals, and his media were sometimes oil and sometimes tempera.
The Kalevala is an immense poem, but here is information regarding it and Gallen-Kallela's works. For the images below, I follow the sequence found there.
Kalevala paintings by Gallen-Kallela (along with many others of his works) can be seen in the Ateneum art museum in Helsinki. If you happen to be in town, don't miss visiting it.