Much has already been spoken of Garro. The former Battle-Captain of the Death Guard. The Captain of the Eisenstein. The person who brought word of Isstvan III to the Imperium. The one who bent his knee of Malcador and became the first of the Knights-Errant. The one who would gather the other unique Knights to his banner.
Garro is fundamentally equipped to be a close combat challenger. His sword, Libertas, is strong with +1 S, but the AP=3 is poor despite the rending ability. The special rule Edge of Truth on Libertas means that he will frequently win combats though with a bonus to the resolution.
His invulnerable save is worth mentioning. Usually it is 4+, but in a challenge, it increases to 3+ which is about as amazing as it gets anywhere in 30k.
A little bit like Loken, who Garro would later recruit!, he can come back from his last wound thanks to his nascent faith in the Emperor on the basis of an Ld test.
All taken, these rules means he is good in close combat and challenges (but not superlatively so), and a reasonable survivor (despite lacking the Death Guard astartes rules or any toughness bonus).
It is also worth mentioning very briefly his falsehood (nice deep striking!) and his assault 3 bolter which is even better thanks to rending.
Overall, Garro is somewhat bland. He's not going to be capable of taking on some of the bigger names in the Horus Heresy. But he will hang around a long time thanks to his faith.
I'd be tempted to take Garro with almost any of the Oaths of the Moment rules. He is certainly capable (just) of succeeding in Headsman, and it would be very fluffy to try for the King Slayer option as well. The others are all perfectly viable as well.
He will do well in most games, but he's either going to be used as a solo-distraction, or as someone to embed alongside the main force.