Gap wedge/ Lob wedge.
How useful would you say they are, and do you think it is worth buying them if you are a reasonably high handicapper?
For the gap wedge It all really depends on your setup, you would have to look at what sort of degrees loft you have between your pitching wedge and your sand wedge. For example my pitching wedge is 47 degrees and my sand wedge is 54 degrees so to put a gap wedge in between those two clubs is hardly worth it, whereas if you have a modern set with a pitching wedge that is 45 degrees and a sandwedge that's 56 degrees then it could well be beneficial to get maybe a 50 degree gap wedge.
It also depends on where you play, I used to carry a 47 degree PW, 51 degree GW, 56 degree SW and 60 degree LW at my old course, as it was fairly short and I didn't need as many long irons woods etc. Now at my new course I have dropped the GW which has been replaced with my 3 iron and put a 54 degree SW in the bag as it is a far longer course.
I'm not sure how beneficial having either clubs are when you're still a high handicapper, it could just complicate things further. Maybe get used to your current clubs first before considering more wedges. You can do more or less everything still with just the PW and SW.