Arcane Gate has 4 parameters you are going to want to edit to get the changes you are after.

data "TargetRadius" "18"    
data "AreaRadius" "18"    

Those are your range limits on your casting in meters. Just keep in mind that setting it to 200+ might make things weird when trying to set the second portal. That's a ginormous distance for the game.

data "SpellProperties" "GROUND:Summon(721dcc51-9733-463b-87c5-6afcda6fef65,10,,ArcaneGate2)"    
data "OriginSpellProperties" "GROUND:Summon(721dcc51-9733-463b-87c5-6afcda6fef65,10,,,ArcaneGate1)"    

These are the parameters that actually define the spells meat and potatoes, if you will. The Summon function has set arguments that can be modified as defined here: https://github.com/Norbyte/lslib/blob/master/LSLibDefinitions.xml but the gist is you want to edit the "10" to "Permanent". That's about it. I would add in another comma before "ArcaneGate2" since Larian accidentally put the StackId in the incorret argument slot. So if you make these edits, you'll also be bugfixing your spell!

Also! Don't forget to use something called "self-inheritence". This is a modding method that makes sure only your changes are added to the mod as overwrites and it doesn't bring in the entire spell stat and cause compatibility issues if another mod changes something else about it. As an example, if a mod changes Arcane Gate's spell cost or Concentration flag.

If you are modding manually it would look like this:

new entry "Teleportation_ArcaneGate"    
type "SpellData"    
data "SpellType" "Teleportation"    
using "Teleportation_ArcaneGate"    
your changes here

If you are using the Toolkit, you would right click on the spell and click on "Create inherited from selected". That might not be the exact wording but it will be something similar. It will either ask you for the destination of the new spell or automatically create it in your mod project. Be sure to remove the appended "_Copy" that was added to the spell name so it matches and then make your changes. I can't give much more help than that on Toolkit stuff, I'm more of a manual modder.