Okay, so looking at each Pokémon individually:
Venusaur
Frenzy Plant, although powerful, isn't the greatest move for in-game usage because of the recharge factor and low PP.
Something like this would probably work better:
- Razor Leaf
- Sludge Bomb (TM found in Rocket Warehouse)
- Leech Seed
- Sleep Powder
Raichu
Raichu gets access to some decent move tutors in FRLG, so with those in mind here's what I would suggest:
- Thunderbolt
- Brick Break (TM found in S.S. Anne or Celadon Dpt Store)
- Body Slam / Quick Attack (Body Slam needs a move tutor)
- Thief (TM found in Mt. Moon)
Brick Break helps to deal with Ground/Rock Pokémon like the Geodude line and Onix super effectively, which you otherwise wouldn't be able to touch with all Electric moves. Thief is a handy move so that you have a Dark-type attack, and is good for snagging held items from wild Pokémon.
Dugtrio
Do you use Dig at all while in caves? If not, and if you don't mind carrying Escape Rope, I would recommend this set:
- Earthquake
- Rock Slide (Move tutor)
- Slash
- Rock Smash
I would recommend Aerial Ace since it covers its Grass-type weakness, but I see you've already used it on Pidgeot :x So I suppose Rock Smash will have to stay for HM usage + type variety.
Pidgeot
Pidgeot is unfortunately pretty limited, but it's not the worst. Its moveset is pretty simple.
- Wing Attack
- Steel Wing
- Quick Attack
- Fly
Steel Wing offers type coverage against Rock-types, which Pidgeot is weak to.
Primeape
I used Primeape just by chance in my FireRed Version too, and that thing surprised me with its crazy strength. This is a set I would recommend:
- Cross Chop
- Earthquake (TM found after defeating Giovanni)
- Rock Slide/Rock Tomb (Move tutor/TM)
- Strength
Gyarados
Unfortunately, Gyarados suffers from the physical/special split that was present in Gen III. If you're looking for a good Water-type for this generation, I seriously recommend using Lapras with Surf, Ice Beam, Thunderbolt, and Psychic. Lapras even learns Ice Beam via level up at level 31, so you don't have to waste your TM on it.