Code G in box 7 with the IRA/SEP/SIMPLE box marked indicates an IRA to 401(k) direct rollover. This suggests that the account at your former employer was a SEP-IRA account or a SIMPLE IRA account; SEP and SIMPLE IRA plans are types of IRA-based employer plans.
A Form 1099-R with code G and the IRA/SEP/SIMPLE box marked should result in the box 1 amount being included on Form 1040 line 15a and excluded from line 15b. Even if this is what your Form 1040 shows, because IRA to 401(k) rollovers are less common than other rollovers, the IRS is asking for proof that the rollover was completed since 401(k) plans do not report receipt of rollovers. The proof would be in the form of an account statement or transaction statement from your 401(k) plan showing receipt of the rollover.
