When I built my pistar, BM defaulted to Network1; DMR+ defaulted to Network2; DMR2YSF defaulted to Network3. My understanding is we can configure up to 6 networks simultaneously.....when I wanted to implement TGIF, everything I found had it set for Network4 and yes, you need to configure these other networks via the Expert menu and edit/paste it in. As for the "7", that is the default prefix added on to DMR2YSF again you can see that in Expert. Here is the Network section from mine....also notice that for TGIF I have mine changed to Prime....
As for how the rewrite rules work, see here:
https://github.com/g4klx/DMRGateway/wiki/Rewrite-Rules
[DMR Network 1]
Enabled=1
Address=74.91.118.251
Port=62031
TGRewrite0=2,9,2,9,1
PCRewrite0=2,94000,2,4000,1001
TypeRewrite0=2,9990,2,9990
SrcRewrite0=2,4000,2,9,1001
PassAllPC0=1
PassAllTG0=1
PassAllPC1=2
PassAllTG1=2
Password="password"
Debug=0
Id=your id here
Name=BM_United_States_3103
[DMR Network 2]
Enabled=1
Address=168.235.109.210
Port=55555
TGRewrite0=2,89990,2,9990,1
PCRewrite0=2,84000,2,4000,1001
Password="PASSWORD"
Debug=0
Id=your id here
Name=DMR+_IPSC2-QUADNET
[DMR Network 4]
Enabled=1
#Name=TGIF_Network
Name=TGIF_Prime_Network
PCRewrite1=1,4009990,1,9990,1
PCRewrite2=2,4009990,2,9990,1
TypeRewrite1=1,4009990,1,9990
TypeRewrite2=2,4009990,2,9990
TGRewrite1=1,4000001,1,1,999999
TGRewrite2=2,4000001,2,1,999999
SrcRewrite1=1,9990,1,4009990,1
SrcRewrite2=2,9990,2,4009990,1
SrcRewrite3=1,1,1,4000001,999999
SrcRewrite4=2,1,2,4000001,999999
#Address=tgif.network
Address=prime.tgif.network
#Password=passw0rd
Password=password
Port=62031
Location=0
Debug=0
Id=your id here
[DMR Network 3]
Enabled=0
Name=DMR2YSF_Cross-over
Id=your id here
Address=127.0.0.1
Port=62033
Local=62034
TGRewrite0=2,7000001,2,1,999998
SrcRewrite0=2,1,2,7000001,999998
PCRewrite0=2,7000001,2,1,999998
Password=PASSWORD
Location=0
Debug=0