Any effects that modify a creature's power and/or toughness without setting them to a specific value (i.e. ones that don't affect base power and/or toughness) will apply after its base power and toughness are set, regardless of the order those effects were created. The same is true for counters that modify its power and toughness.

As Galion's ability resolves, the base power and toughness of the targeted creature are set to Galion's actual power and toughness, not its base power and toughness. If Galion's power or toughness changes later in the turn, the other creature isn't affected.

Galion's ability overwrites all previous effects that set a creature's power and toughness to specific values. Other effects that set its power or toughness to specific values that start to apply after the ability resolves will overwrite this effect.