Check the mod priority guide:
https://docs.google.com/spreadsheets/d/1ty239kyJbBs1sLhTo1JvSWuN0mIJsK1nwVdI6OLLtXo/view?rm=minimalGepard M1 has been a mostly forgotten doll. Now armed with her upgrade, she carves out a decent niche for herself as a softer-hitting but permanent uptime IWS-2000 with a fancy instadeath flair.
Her new Skill 2 is
Pact with Death. She will start by prioritising enemies that have not been attacked yet. This is done as all her attacks apply a Mark of Death to the enemy. When a non-elite foe with this mark reaches 30% HP, they just die. Nothing fancy, straight into the ground and out. On top of this, while her Skill 1 is charging (available to use), she increases her Damage substantially while reducing her Rate of Fire.
Her Skill 1 always remains an option though, if you happen to need that immediate high burst of damage. Sadly, do note that due to her Skill 2 condition, this means she loses the immense Damage boost prior to the Interdiction Shot.
All in all, Gepard M1 MOD carves out an interesting and unique niche for herself. The ability to eliminate enemies earlier than usual and make herself a permanent uptime IWS is quite fun to use. Pretty good stuff for a previously unremarkable doll.
MOD2 is what you want mostly, though MOD3 does come with a decent Special Equipment.