When filing an ICAO flight plan with LPV, is marking GNSS equipment necessary or redundant?
You should fill both G and B.
If I understand the last row of the official ICAO guidance correctly, field 10 (equipment code) of the flight plan must contain at least B and G if you want to fly LPV approaches:

(cropped from ICAO Guidance for the provision of NAV/COMSUR information in the New ICAO 2012 Flight Plan)
You will also have to enter &quot;NAV/ SBAS&quot; in field 18.
This is also confirmed by the following information on the ForeFlight support page:

When specifying a PBN code, be sure to also specify R (Code for Performance-Based Navigation (PBN) Approved) and the associated sensors that the PBN code requires. For example, PBN Code D2 is RNAV 1 using GPS, therefore Equipment Code G (GNSS) must also be specified.
[...]
These tables are a general aid for entering filing codes in ForeFlight and are not an authoritative or comprehensive equipment specification. Consult the AFM or AFM supplements for your aircraft to determine which codes are actually appropriate before attempting to file using ForeFlight. The codes listed below are in addition to the other ICAO Equipment code that may be appropriate for your aircraft.
WAAS

GNS 430W - R, B, and PBN B2, C2, D2
GNS 530W - R, B, and PBN B2, C2, D2
GNS 480 - R, B, and PBN B2, C2, D2
GTN 6XX/7XX - R, B, and PBN B2, C2, D2
GPS 175 - R, B, and PBN B2, C2, D2
GNX 375 - R, B, and PBN B2, C2, D2
G1000, 2000, 3000, 5000 WAAS - R, B, and PBN B2, C2, D2


(foreflight.com, emphasis mine)