Pretty sure it's slaved to the EDI, isn't it? If so:
(edit: someone else said no. anyways, while we're talking about it, here's your EGI in-flight alignment info)
CDU -> SYS -> NAV -> INS -> ALIGN
On the right side, you should have a few options. One is GROUND, which is how you start your stationary/parked alignment... and just below it, is your INFLT alignment!
From the DCS A-10C Flight Manual EN, page 191:
See also, right under it (both in the manual and on the CDU screen)