That God gave (hoti edōken ho theos). Declarative hoti in apposition with marturia as in 1Jo 5:14; Joh 3:19. Note aorist active indicative edōken (from didōmi) as in 1Jo 3:23., the great historic fact of the Incarnation (Joh 3:16), but the perfect dedōken in 1Jo 3:1 to emphasize the abiding presence of God’s love.
Eternal life (zōēn aiōnion). Anarthrous emphasizing quality, but with the article in 1Jo 1:2.
In his Son (en tōi huiōi autou). This life and the witness also. This is why Jesus who is life (Joh 14:6) came to give us abundant life (Joh 10:10).