|This build is meant to fully optimize Gordin to his absolute maximum potential at this point in the game. As such, this is a build for those striving to +10 Gordin, S-rank Summoner Support him, and utilize him in all game modes effectively, both during player and enemy phases.
At 40+10 S-rank, with an Atk boon, plus a Res-boost with weapon refinement, Gordin has a respectable 51 attack, 51 HP, and 38 defense, with a less respectable 31 speed and 26 res. (While you don't need speed if you're taking little to no damage from most enemies, a +Spd IV gives Gordin a superboon, making his speed cap out at 35. If you run a refined Marth with Drive Speed, or a Performing Arts Azura, or really just S support Gordin with anyone who primarily buffs speed, this becomes an absolutely viable IV, on par with +Atk due to its ability to prevent doubles from some stronger mages/dragons. However, unless you're also buffing Gordin's attack, it would be wise to run Ignis full-time with a +Spd IV.)
An unbuffed Gordin (with DD3 seal) will be reaching 38 Def against non-ranged physical units, 44 Def against ranged physical, 32 Res against mages, and an unfortunate 26 Res against dragons. However, this is where Clarisse's Bow comes into play. Essentially giving Gordin a ranged form of slightly-weaker refined Dark Breath, now this best-boy can be used to effectively bait all ranged and most non-ranged physical units (and survive most anything that isn't a +10 dragon/special-cooldown-reduction unit/full-buff-Bladetome) with ease, and then attack them, and any of their nearby allies, with far, far higher survivability due to the atk/spd debuffs, as well as inching closer to his Aether or Ignis proc, which nearly always kills. (Swapping Clarisse's Bow for Guard bow allows him to essentially bait all ranged units to death, as well as offering the chance for a Def refinement, but greatly lowers his effectiveness during player phase and against dragons. Also of note, using Guard Bow + a Close Defense 3 seal is not recommended, as his Res still winds up being slightly subpar against strong mages, alongside losing the AoE atk/spd debuff that can prevent those mages from doubling him.)
His A Skill, Close Counter, speaks for itself, and is what allows him to be effective against all units during enemy phase. His B Skill, Cancel Affinity 3, is primarily for Arena, whereas I find his on-hand Vantage 3 works best in the other game-modes. (More so than QR3, especially with speed buffs or a +spd IV) His C Skill should either be Threaten Def 3 or Threaten Atk 3 (with Guard Bow). I personally find Threaten Def 3 to be most effective, as it usually allows him to kill anything that survives a round of combat with him, and also boosts the overall damage of his specials. Threaten Atk 3 is redundant with Clarisse's Bow, but if you prefer Guard Bow, it would probably be the best choice to help with dragons. Of course, this is the most flexible skill he has, and his overall effectiveness won't be damaged when tailoring it to your likes.
As mentioned earlier, a solid S-Support partner that deals in mass buffing, such as Performing Azura (any dancer with Geyser Dance seems just as good) or Refined Marth specifically (or both, if you really want to go overboard and try to push all his stats above 40 on enemy phase), will eliminate any remaining worries (dragons and even special-cooldown-reduction units won't kill him one-on-one, thanks to his 51 HP) that Gordin could have in battle, exempting ever-threatening fully buffed bladetome units.
Though this buff and bait strategy is very effective, the true strength of this build comes with the diversity of strategic options. Gordin can attack any unit that cannot counter him, and plenty that can thanks to his defense, on player phase. His attack is high enough to do decent damage on most units, and he'll also instantly debuff them and all around them. This serves to make him even more crazily effective on the next enemy phase (essentially giving him +5 spd/def/res against the debuffed units). Reposition allows him to become a living wall for anyone he moves out of trouble's range. His status as archer lets him OHKO some of the most dangerous fliers in the game, and his high attack sometimes OHKOs fliers even when they have Iote's Shield equipped. With a basic +4 Res buff, his Clarisse's Bow build will allow him to survive most if not all Reinhardts, and he's already a natural Bow Lyn counter as well.
Lastly, this build's specific weaknesses bear mentioning, though a lot of them revolve around strategic missteps rather than 'inevitable weaknesses'. Avoid baiting characters specifically built to pile on damage, such as a +10 Cherche/Seigbert/F!Celica. Usually, these foes have Death Blow/Swift Sparrow and brave weapons, or a firesweep weapon, so they won't be able to counter Gordin's debuff on player phase, making them much easier to deal with when you hit them first. +10 Mia in particular is difficult for an unbuffed Gordin to handle, but using the tools at your disposal; debuffing/dancing/buffing, will usually allow her to be killed before she can attack Gordin. (And if she's able to counter, then she won't be running her cooldown reduction A skill, making her far easier to deal with through baiting) Anything with Bold Fighter should be hit by Gordin's debuffs first, but if he's on a defense tile then he can usually hold his own (unless the enemy is running Galeforce). Baiting armors 'without' Bold Fighter is almost always the best choice in high level arena matches. Halloween Henry and dragon units can be troublesome, but not always; as such, a Falchion user can be extremely good as a partner. Raventomes without TA and with bowbreaker are Gordin's biggest (possibly only 'true') weakness, but they're so specifically built that any non-colorless character with high res can deal with them instead. Bladetomes should be broken away from their buffing friends before combat, as always.
Needless to say, if you're in an arena match that has defense tiles, with this build and a buffing S support friend, Gordin will rarely take any real damage.
~Shota is justice.~