Realistically, Garp might be stronger than Blackbeard at the moment if we assume old Garp > old Rayleigh which is fair I think.
i think garp's character is he's frustrated that dragon didn't follow his footsteps and become a legendary marine
then he tried to turn luffy into a marine but luffy was corrupted by shanks
this hurt him (he could never have the father / son relationship he wants) and he kinda gave up until he met coby
finally he finds a marine son, and coby has so much potential
what i'm getting at is that garp going to blackbeard's hideout doesn't necessarily mean he can defeat blackbeard (though let's not rule this out)
it could mean that as long as he can save coby, he's ok with not making it out alive
having said this
ODA YOU PIECE OF SHIT
DON'T YOU DARE KILLING GARP