It is similar in that they use characters from their alphabet as numerals but not exactly the same way as the Romans. Greek numerals are decimal, based on powers of 10, just like Roman and Arabic. The units from 1 to 9 are assigned to the first nine letters of the old Ionic alphabet from alpha to theta. Instead of reusing these numbers to form multiples of the higher powers of ten, however, each multiple of ten from 10 to 90 was assigned its own separate letter from the next nine letters of the Ionic alphabet from iota to koppa. Each multiple of one hundred from 100 to 900 was then assigned its own separate letter as well, from rho to sampi.

Not really, they're based on gematria like Hebrew numerals. α = 1, β = 2, γ = 3 and so on

Roman got it from the Greek too before adapting it, iirc.