Here is the full list of all 42 STEINS;GATE achievements. It takes around 40-50 hours to unlock all of the achievements on Windows.
[Phone Trigger] Replied to an email.
[Phone Trigger] Viewed email send history at least once.
[Phone Trigger] Read an email.
[Phone Trigger] Changed phone wallpaper.
[Phone Trigger] Changed ringtone.
Prologue Clear
Started Steins;Gate.
Unlocked all Achievements.
Chapter 1 Clear
Chapter 2 Clear
Chapter 3 Clear
Chapter 4 Clear
Chapter 5 Clear
Chapter 6 Clear
Chapter 7 Clear
Chapter 8 Clear
Chapter 9 Clear
Reached Steins Gate.
Made friends with Kurisu via email.
Reached Kurisu's ending.
Reached Mayuri's ending.
Reached Suzuha's ending.
Reached Luka's ending.
Reached Faris's ending.
Completed all endings.
Unlocked 25% of CGs.
Unlocked 50% of CGs.
Unlocked 75% of CGs.
Unlocked all CGs.
[Phone Trigger] Cleared someone's mail without reading it.
[Phone Trigger] Ignored a phone call.
[Phone Trigger] Received all 13 RaiNet episode summaries.
[Phone Trigger] Received all Gero Froggy wallpapers.
[Phone Trigger] Received 4 songs from a chaotic head.
[Phone Trigger] Received 2 Upa wallpapers.
[Phone Trigger] Received 3 wallpapers of characters from a chaotic head.
[Phone Trigger] Asked by Kurisu to buy cup noodles twice in a row.
[Phone Trigger] Learned why Mayuri started making costumes.
[Phone Trigger] Received an unusual proposition from Suzuha.
Learned about Luka's relationship to his sister.
[Phone Trigger] Offered tea by Faris.
Made 10 mistakes while attempting to set the PhoneWave (name subject to change)'s timer.