Ganryu infamously lacks a safe i10 move, being confined to his b+1+2 headbutt that's -13 on block. This means that Ganryu must respect a lot of Yoshi's pressure -- particularly in DGF. For this reason, most Yoshi players, even people who prefer to stay in NSS for other matchups, elect to stay in 1SS against Ganryu.
3~4 forms the backbone of Yoshi's gameplan against Ganryu due to the amount of safe options Yoshi has from it, and the fact that Ganryu must take disproportionately higher risk to address any of them. Abuse 3~4 (u) as much as possible.
DGF Mixups
On block, with the DGF transition, Yoshimitsu's (correctly spaced) 3~4 leaves him at +5-6, meaning that Ganryu has no safe way to interrupt Yoshi's i16 DGF 2 which i a safe launcher with enough frame advantage to let you safely get out of DGF if need be, or keep up DGF pressure.
You can't just spam it as it is a high that he can duck -- but that's where the mindgames come in:
DGF 4
If Ganryu's been conditioned to duck after 3~4 into DGF, you open him up to getting hit by DG4, a safe mid that nets you a free mixup on normal hit and a launch on counter or airborne hit.
DGF 3+4
If Ganryu's been conditioned to freeze after 3~4 into DGF, he opens himself up to getting hit with the high, unblockable DGF 3+4 which will give you favorable oki. It also floor breaks on Forgotten Realm into a free combo.
No transition
To add another layer into the mindgames, if Ganryu been conditioned into interrupting with b+1+2, you can forego the DGF transition altogether to bait it out and either do an i13 punish, or duck and launch.
Lastly, despite even though 3~4 without the transition is -6 to -5 on block, Yoshi doesn't give up his turn on account of him having an i6 launcher in Flash (1+4) -- if Ganryu pushes a button he gets launched.