Looking into this. @Sure Lock Homes I ripped the materials from that exact workshop addon and updated our content pack and it fixed it for me but I'm not sure why it isn't working for everyone.
I updated the M4A4 texture earlier today as to not conflict with the M16 texture. After restarting your GMod to apply that fix, are the hands still missing?
This gun's arm model calls for "tm_arms_leet.vtf" "materials\models\weapons\v_models\arms"
They work with My Dime Is Up's workshop addon enabled, but it might be that for someone joining the server without being subscribed to any of the addons the arms don't seem to auto download.
Using the command "mat_texture_list", clicking "Show ALL Textures", then selecting "Where Is" on "tm_arms_leet.vtf" it says: 'Giant's Lair Modded TTT Weapons' - D:\SteamLibrary\steamapps\workshop\content\4000\3489487718/gmpublisher.gma
Also the textures "rif_m4a1.vtf" & "rif_m4a1_exponent.vtf" for the M4A4 are still located in "Giant's Lair Weapons"
'Giant's Lair Weapons' - D:\SteamLibrary\steamapps\workshop\content\4000\3554565630/gmpublisher.gma
With the fix from @uintancæstir and some worksho updates, can you confirm if this issue is still present following the rollout of the April Update Package? I'll close this if this is no longer an issue or there is no response within the next week.