Here is the proper listing of gamemodes and gametypes.
# [Game Modes] gametype gamemode mapgroup
# Arms Race 1 0 mg_armsrace
# Classic Casual 0 0 mg_casualsigma, mg_casualdelta
# Classic Competitive 0 1 mg_active, mg_reserves, mg_hostage, mg_de_dust2
# Custom 3 0
# Deathmatch 1 2 mg_deathmatch
# Demolition 1 1 mg_demolition
# Wingman 0 2
# Danger Zone 6 0 mg_dz_blacksite (map: dz_blacksite), mg_dz_sirocco (map: dz_sirocco)
You need to place mp_give_player_c4 1
in your gamemode_x_server.cfg
where x = gamemode. E.g. gamemode_casual_server.cfg
You do not place it in server.cfg, you do not place it in gamemode_casual.cfg. You place it in gamemode_casual_server.cfg
If it doesn't work that way, a plugin is messing with it.