Need Help with Baby Clothes Sizing

jamieandlori

We just had our baby shower on Saturday.  I have spent today organizing clothes into sizes and trying to figure out what else we need.  I'm a FTM and I feel completely overwhelmed!  How does sizing work? For example, are 3 month clothes going to lean more toward 0-3 sizing or 3-6 month sizing?  I know it will depend on brands, so if anyone can help with brand sizing that would be awesome, too.  Also where does GAP's size 6-12 months fit in the sizing scale?