Well, because youtube is part of google, so they have lot of ip's
NetRange: 74.125.0.0 - 74.125.255.255
CIDR: 74.125.0.0/16
OriginAS:
NetName: GOOGLE
NetHandle: NET-74-125-0-0-1
Parent: NET-74-0-0-0-0
NetType: Direct Allocation
RegDate: 2007-03-13
Updated: 2007-05-22
Ref: http://whois.arin.net/rest/net/NET-74-125-0-0-1
OrgName: Google Inc.
OrgId: GOGL
Address: 1600 Amphitheatre Parkway
City: Mountain View
StateProv: CA
PostalCode: 94043
Country: US
RegDate: 2000-03-30
Updated: 2009-08-07
Ref: http://whois.arin.net/rest/org/GOGL
OrgTechHandle: ZG39-ARIN
OrgTechName: Google Inc
OrgTechPhone: +1-650-253-0000
OrgTechEmail:
OrgTechRef: http://whois.arin.net/rest/poc/ZG39-ARIN
You could create a rule that its form * * * 74.125.0.0/16 * "your another gateway" * *
in that "your another gateway" you can select that under Advanced features->Gateway
That should do that, but only con is that everything what you send to google is going to use only one gateway(so much the redudancy)
or you can use only this address: 74.125.224.192 for the youtube.com
EDIT:
OH forgot, if you're going to make also other streaming sites, and you like to have low number of rules doing the same job. you may create alias (ex.streamsites) and under that you can add all the ip's you like to handle via one gateway rule.