not a fan of strict sideboard guides and i anticipate that the exact list will change by at least a few cards anyway (e.g. for denver i might swap some sb removal for dec in stone or skysovereign, etc.)

vs zombies:

matchup is about racing and they are weak to fliers. harvester is the best card against them - ballistas - gideons + cuts, abrades, harvester.

vs mono red:

stay alive and eventually all of your cards will be better than theirs.  they often can't afford to play around avacyn so it usually just wrecks them.  - gideons, hearts + harvester, cuts, abrade.  i expect red builds to become bigger over time so might be worth considering cutting 1 scrapheap to leave in a heart vs glorybringer and chandra

vs gb:

your 1 toughness creatures are liabilities vs ballista and liliana.  they have trouble beating planeswalkers.  switch to a planeswalker control plan and try to keep the board clear and then resolve gideon or chandra.  - exemplars, ballistas, harvesters, avacyns + all planeswalkers and removal (including doomfall).  avacyn is ok but not great vs them and is a liability vs gonti

vs mardu:

matchup is about whoever can keep a gideon in play.  switch to planeswalker control mode.  - exemplars and harvesters + planeswalkers and removal (no doomfalls).  avacyn is one of the best cards in the matchup.

vs temur energy:

gideon is medium vs virtuoso and glorybringer.  this matchup is difficult and at the moment i think it's best to try to race in the air.  - gideons, pushes + heart, harvester, removal (cut // ribbons)

vs control:

assuming blue control here

- pushes, ballistas + planeswalkers and doomfalls.  avacyn is medium but at least you can play it as an instant to tax their mana.  if you have to choose between resolving something when they have 4 mana vs when they don't, better to go for it when they have 4 mana.  it's usually really important for them to resolve a glimmer asap (to draw more answers but also just to set up their gearhulks on curve) so forcing them to choose between doing that and countering a planeswalker can be huge.  disintegration can rot in your hand sometimes but you don't want less than 4; it often just ends the game on the spot vs a gearhulk and is sometimes necessary if they are siding in thing in ice.

vs uw monument

not a popular deck atm but wanted to mention it here because gideon is kind of meh vs them.  they are great at chump blocking and have fliers to pressure it.  better to side and your flying vehicles and race in the air.