The only problem is that Subscript[] doesn't accept the invisible comma. (I'll leave that explanation for the experts). With a "visible" comma the code works as expected.

Subscript[g, j, k] /. {j -> 2, k -> 2, Subscript[g, 2, 2] -> 3}

with a value of 3 for the expression (but you didn't assign it to anything). Symbols formatted with visible or invisible commas in the subscript are the same symbol. Your other code should work.

NB: In the documentation section "tutorial/MakingDefinitionsForIndexedObjects" is the statement

You can think of the expression a[i] as being like an "indexed" or "subscripted" variable.

They are also problems converting them to strings, as the subscript is turned into a line feed.

Thanks for the fast replies. I didn't find any hint to this in the Mathematica documentations about this issue. And of course, I didn't know either that I hit a "hot topic" by chance. Anyways, the thing is that I am trying to calculate Christoffel symbols for some coordination conversions. The subscripts and the superscripts are essential in the classical form of the equations and they keeps messing up my calculations. It seems that I have to find away around those trouble makers.

Thank you again