I found the way to get Gauntlet lit when mouse1 is pressed.
I went to .../def/weapons/gauntlet.def and found these statements:

entityDef weapon_gauntlet
{
...................... // not important for us
// ---------------------------- Muzzle Flash ---------------------------------------

"mtr_flashShader" "lights/muzzleflash"
"flashColor" "0.7 0.8 1"
"flashRadius" "200"
"flashPointLight" "1"
"flashTime" ".1"
"flashViewOffset" "15 5 -10"
......................
}

I found similar shit in every weapon definition file but they didnt miss fx guides like that (for lighting gun):

"fx_impact" "effects/weapons/lightninggun/impact.fx"
"fx_impact_mp" "effects/weapons/lightninggun/impact_mp.fx"
// "fx_impact_water" "effects/ambient/splash_med.fx"
"fx_trail" "effects/weapons/lightninggun/trail.fx"
"fx_trail_world" "effects/weapons/lightninggun/trail_world.fx"
"fx_crawl" "effects/weapons/lightninggun/crawl.fx"
"fx_spire" "effects/weapons/lightninggun/spire.fx"
// "fx_flash" "effects/weapons/lightninggun/muzzleflash.fx"
// "fx_flash_world" "effects/weapons/lightninggun/muzzleflash_world.fx"
"fx_respawn" "effects/mp/itemrespawn.fx"
"crawlTime" ".05"

Compare with such a list for a gauntlet:

"fx_impact" "effects/weapons/gauntlet/impact.fx"
"fx_impact_flesh" "effects/weapons/gauntlet/impact_flesh.fx"
"fx_impact_monstermetal" "effects/weapons/gauntlet/impact_monstermetal.fx"

So it seems like its possible to add "muzzle flash" for a gaunlet because the possibility is nested in def file and I dont need to recompile dlls to do that