Other gain antenna are available, information on request. ALWAYS check with your local authority that you have the approval to use a gain antenna. In the UK you MUST have a licence from Ofcom.
Very roughly (and we apologise to all those antenna experts, this advice is rule of thumb only!) when connected to a 10W Epic - a Unity gain antenna means there is no gain at all, a 3db gain antenna can give you roughly double the power i.e. 20W and a 6 db gain will double that to around 40 W.
Ofcom are only interested in ERP that is radiated power from the antenna not what the modem itself is giving out. The radiated power from the antenna will take into account any RF loss made by connectors or length of cable specialist equipment is required to measure this accurately. Recent changes in rules mean that you can now apply for higher power than before - but as they are forever changing the rules this should be checked before purchase!