Are you referring to some specific clause of the GPL? Using the exception you have to abide to the GPL for everything but having to provide your own source code.
Anyway there is this clause in the GPL3:
" If you convey an object code work under this section in, or with, or
specifically for use in, a User Product, and the conveying occurs as
part of a transaction in which the right of possession and use of the
User Product is transferred to the recipient in perpetuity or for a
fixed term (regardless of how the transaction is characterized), the
Corresponding Source conveyed under this section must be accompanied
by the Installation Information. But this requirement does not apply
if neither you nor any third party retains the ability to install
modified object code on the User Product (for example, the work has
been installed in ROM)."
The Source requirement is not applicable because the Exception text and the clause specifically states that you don't have to provide installation information if the product is not modifiable by anybody including you (ROM).
It the micro can be erased and re-flashed then you should consider this part:
" When you convey a covered work, you waive any legal power to forbid
circumvention of technological measures to the extent such circumvention
is effected by exercising rights under this License with respect to
the covered work, and you disclaim any intention to limit operation or
modification of the work as a means of enforcing, against the work's
users, your or third parties' legal rights to forbid circumvention of
Basically you cannot forbid your users to hack and re-flash their devices even if you don't have to provide your own source code except the ChibiOS/RT code itself (and this requirement is satisfied by simply giving a link to the ChibiOS/RT pages.
Alternatively you may consider some of our commercial licensing options