It's very hard to gain weight on nuts, especially raw nuts. Forget everything you've been told about how bad animal based fat is, that's the only fat that's natural and paleo in large quantities. You've already cut out eggs and rejected dairy (as most paleoers do), so that leaves meat fat/marrow/suet/brain. Get yourself some good quality sources and use that for your fat.
Also pescatarian tends to make it harder to gain weight, as most fish meat is too lean. There are some fatty fish out there, where I live it's generally river fish, but they are more polluted than ocean fish. You'll have to experiment and research to find the types of fish that are good for you.
Keto also makes it hard to gain weight, particularly fat weight. Have some raw unheated honey for carbs if you need to add some fat weight.
Read through this forum and you'll find this is much more than your average paleo diet forum, with a wealth of astounding knowledge, thoughts and discoveries that go far above and beyond everything you've heard elsewhere.