This is the official timeline of Option Gamma stories.
I personally recommend reading Option Gamma first, even once Threshold is completed (Currently on Hiatus). Threshold is/has been/will be written to be read as the sort of prequel you read *after* its sequel(s).

Be warned that if you read anything dated contemporary to, or after, Option Gamma or Genesis, that major universe plot points will be spoiled for you. If you read in the given order here, as far as I am aware, you will not have anything spoiled for you.

The timeline includes AD and AC (after contact) Dates, as well as the introduction of each Conversion Program in parenthesis.

*2102 AD [00 AC]: -Threshold
*2104 AD [02 AC]: -Threshold contd. (Bureaus Open, Ponification Introduced)
*2114 AD [12 AC]: -Option Gamma (Gryphonization Introduced)
-The First Choice
*2115 AD [13 AC]: -Balance
-Momentum
-State of Mind (Draconification/Zebrification Introduced)
*2116 AD [14 AC]: - (Buffalo & Diamond Dog Programs)
*2117 AD [15 AC]: -Genesis

[To Be Continued]