i dont know how you play gdos, but i think it has good matchups against sp decks if you play mew. if you manage to get a gdos into a lost zone, you can basically get a prize every turn. you only need 1 less prize to use mew effectivly. against dialgachomp you can´t obviously use mew, but i think gdos doesnt even need a mew to beat dialgachomp. however, magnerock(or magnezone/champ/rock whatever) should beat gdos easily.
@mike
i think people dont play magnezone, because they keep on their successful decks. Magnezone+Machamp is a good deck and i think it has good matchups against all good decks , esp. sp, if both decks have an equal start.
my tier list
tier 1:
luxchomp
dialgachomp
gdos
magnezone/machamp/regirock
tier 2
vilegar
lostgar
arceus
jumpluff
regigas
tier 3
scizor
charizard
steelix variants
not sure about sablelock because almost every deck, which can survive the first 2-3 turns, wins against sablelock