Sideboard for Modern U/G infect against Gideon
There are some common sideboard cards that are effective here.

Spell Pierce can keep Gideons off the field for just 1 mana. Gideons cost a lot of mana (for modern), so they should rarely be able to pay. It can also deal with removal and nasty sideboard cards like Solemnity

Echoing Truth can help manage a resolved Gideon. It can remove a Gideon from the field to turn off his emblem and can remove all the Knight Ally tokens at once (though note that both Inkmoth Nexus and Blighted Agent go around them anyways). It can also handle infect hosers like Solemnity or Melira.

Spellskite is probably the best way to deal with Gideon of the Trials's +1. It also deals with Path to Exile, a common sight in decks that play Gideon.