The I-gate symbols are documented on the YAAC help page on configuring an I-gate. However, as a short reminder, there are several symbol choices, depending on how your I-gate is set up.
I& is the typical I-gate symbol (& is the symbol code for gateways of all types, and the overlay indicates the type).
T& indicates a transmit-capable I-gate (since so many are receive-only).
2& indicates a transmit-capable I-gate that uses a digipeat path allowing a second RF hop (one digipeat).
R& indicates a receive-only I-gate (unable or unwilling to forward Internet packets to local RF stations).
I# is a variant of the digipeater symbol indicating a station functioning as both a digipeater and an I-gate.
Don't use D&, as that indicates a D-star gateway.
Hope this helps.
Andrew, KA2DDO
author of YAAC
