Why do some airfields not have a climb gradient for LPV? Can anyone advise why for some airfields there are minimum approach climb gradients for LNAV with none for LPV, while at other airfields there are minimum approach climb gradients for both LPV and LNAV?

This is an extract from the plate for Campbeltown - EGEC. 



For other airfields, MPA gradients are also given for the LPV as well as LNAV such as Islay - EGPI, shown below.
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.