Why do some airfields not have a climb gradient for LPV?
When it is not stated, it means it is the nominal value of 2.5%, i.e., for the first example, the LPV climb gradient is 2.5%. When an additional OCA/H is published, then the OCA/H and gradient values are published (second example).
From PANS-OPS:

6.2.2.2 Climb gradient in the intermediate phase. The nominal climb gradient (tan Z) of the missed approach surface is 2.5 per cent.
6.2.2.3.2 The OCA/H for the nominal 2.5 per cent must always be published on the instrument approach chart. If additional gradients are specified in the construction of the missed approach procedure, they and their associated OCA/H values must be published as alternative options.