In Path Of Exile, Crit is rolled once per skill activation (an attack with multistrike for example still counts as only one skill activation). Unlike buy poe orbs, this requires you to spend some time to understand.
Think of it this way, when you attack it rolls a number 1 to 100. Say it gets 65. On each enemy it compares that number to your crit chance, and if it’s lower than your crit chance, it crits. If you have assassin’s mark on some enemies, the crit chance on those enemies is different. So if your crit chance is 62, it compares 65 to 62, it’s not lower, so it doesn’t crit that enemy. If a second enemy is affected by Assassin’s Mark, you have a crit chance of 67 on that enemy, so it compares the same 65 to 67. 65 is lower than 67, so it crits.
GMP will increase the odds of a power charge IF you crit, since the percentage chance on PCoC is rolled independently for each crit. But you can only ever get one power charge and one frenzy charge per skill activation.
Cursed enemies have an additional x% chance to receive a Critical Strike: This effect is additive with player’s current chance to deal a critical strike. For example, if a player has a 20% chance to critically strike with an attack or spell, and it hits a target inflicted with level 1 Assassin’s Mark, the target will have an additional 5% chance of receiving a critical strike, for a total of a 25% chance. Because this effect is separate and additive with a player’s critical strike chance, which has a maximum of 95%, Assassin’s Mark makes it possible for enemies to have a 100% chance of receiving a critical strike.
In fact, while critical strike chance is rolled once per attack, accuracy is rolled per hit (note that this is different from per projectile since one projectile can hit multiple targets with pierce, chain and fork). If you don’t hit you obviously don’t crit either, so with gmp increasing your chance for a potential hit you also increase your chance for a power charge.
Also critical strikes roll accuracy twice, first on the initial hit and then again on the critical strike. So if your attack rolls a critical strike it can happen that you still don’t crit the monster cause your second accuracy roll failed. Also pcoc has only a %chance so more projectiles = higher chance. All together GMP will help you get a more reliable power charge sustain (especially with terrible accuracy).
There’s a lot more players guide on U4GM, so be sure to check out if you need some guides. Keeping an eye on poe buy items, you will get more gains.