"giganet" is gigaset.net,
VoIP with Gigaset.net from Siemens Gigaset
Yes, it is possible to specify Outbound Proxy separately for each account. Gigaset phones are working just fine (with voxalot as well) without using outbound proxy.
Depending on the NAT device involved into this setup it may be necessary to configure Gigaset to use a
local SIP port different from 5060 or use a random port (may be found at
Settings - Telephony - Advanced settings).