I Believe that:
IPKall, Sipgate,custom-gv (custom) were from the old way where Google called back. Keep them but check the Disable Trunk about 3 or 4 options down.If we have to go back to the old way - you'll have it ready.
Vitel-Inbound and Vitel-Outbound is for Vitelity which Piaf and Nerdvittles users get a special rate (is this still true, Mr Ward?) via a special sign up link on the
Nerd Vittles website - just scroll down past the article (large Green Square) by using this link NV gets a small referral donation -- helps out the projects.
Freenum is for the ISN (sip direct calling) search this forum for freenum / mothership for some details. I can't be much help on this as I can't get mine working myself yet.
The gvoice and if you have a gvoice-gmailname these are the NEW way.
ENUM is for the e164.org /ENumPlus lookups - If a number is listed on enum.org, e164.org enumplus.org then if a match is found for the number you are calling with a SIP uri route the call goes out as pure VOIP/SIP and bypasses the PSTN altogether.
If you are unsure of a trunk, don't delete it but just Disable it, you may need it in the future -- Disabling turn the Name to a gray background.
Hope this gives you some ideas as to the pre-installed trunks and their uses.
Happy PBXing