Hi Lara

if you want a gender * group interaction, you will have to exactly model that, and thus you would need 3 group variables:
a) gender: (e.g. male = 0, female = 1)
b) group: (e.g. control = 0, group = 1)
c) group * gender: simply multiply a * b

you can still have your other independent effects.

a) captures the pure gender effect, b) the pure group effect and c) their interaction.

You can then test for whichever you are interested in. If you are interested to find out if there is a significant group by gender interaction, then test for column (c)

Martin