You'll find the official tournament regulation here.
At Gothia Cup the age categories G13, B13, G14B and B14B plays 9-a-side.
Rules for 9-a-side
The same rules as in 11-a-side are applicable, note that the offside rule is applicable just as in 11-a-side football. The size of the goals is the same as in 11-a-side football. The size of the field is regulated by the Swedish Football Federation’s rules for ”Large 9-a-side”, i.e 72x55 meter.
Number of players and substitution
In 9-a-side there are nine player on the pitch - one goalkeeper and eight players. Each team may use a maximum of 16 players (nine players on the pitch and seven substitutes) in each match. A substituted player may reenter in the match.
In 9-a-side G13 and B13 plays with ball size 4. G14B and B14B plays with ball size 5.
In 9-a-side, one (1) dispensation is by default given for one overaged player, who can be at most one year over the age limit. Note that it’s not allowed to have more than one overaged player in the roster, who change places in the squad for each match.
Girls may play in boys classes as long as the regulations for age categories are followed.