When you say 3.5 adventure-making or SRD, is that D&D?

Essentially, yes. The three elements we provide, from which the GMs will create their adventures, will be from the 3.5 SRD. However, players and GMs are free to include any other material that works under the OGL rules.

What experience level of GM is this geared for?

Any level. Iron GM measures Game Master brilliance, not frequency or complexity of play. Game Masters shine when they create exciting, original adventures then give their players the game of their lives.