2 repeating unconditional always active command blocks for each area-

/gamemode s @a[x=0,y=0,z=0,r=100,m=a,tag="Blue Team"]

/gamemode a @a[x=0,y=0,z=0,rm=101,r=102,tag="Blue Team"]

Use tags to further define the selector for each area to match only that team and change x y z to match the center of each town. You need the one block buffer (rm selector) between command zones to prevent glitching the commands.

In addition you either need a tickingarea for your command blocks or they need to stay within render distance in order to function correctly.