Since I couldn’t find an official and complete Google reference document, I looked at the current Javascript source code of Gmail/Hangouts.
Ioa = {
"!:)": 128579,
"!:-)": 128579,
"(=^..^=)": 128568,
"(=^.^=)": 128568,
"(N)": 128078,
"(Y)": 128077,
"(]:{": 128115,
"(n)": 128078,
"(y)": 128077,
"-<@%": 128029,
"-_-": 128529,
":''(": 128557,
":''D": 128514,
":'(": 128546,
":(": 128542,
":(:)": 128055,
":(|)": 128053,
":)": 9786,
":)X": 129303,
":*": 128535,
":,": 128527,
":-(": 128542,
":-)": 9786,
":-)X": 129303,
":-*": 128535,
":-,": 128527,
":-/": 128533,
":-D": 128512,
":-O": 128558,
":-P": 128539,
":-S": 128534,
":-\\": 128533,
":-o": 128558,
":-p": 128539,
":-s": 128534,
":-|": 128528,
":/": 128533,
":3": 128568,
":C": 9785,
":D": 128512,
":O": 128558,
":P": 128539,
":S": 128534,
":X)": 128568,
":\\": 128533,
":o": 128558,
":p": 128539,
":s": 128534,
":|": 128528,
";)": 128521,
";*": 128536,
";-)": 128521,
";-*": 128536,
";-P": 128540,
";-p": 128540,
";P": 128540,
";_;": 128546,
";p": 128540,
"</3": 128148,
"<3": 10084,
"<\\3": 128148,
"='(": 128546,
"=(": 128542,
"=)": 128522,
"=*": 128538,
"=/": 128533,
"=D": 128516,
"=O": 128558,
"=P": 128539,
"=\\": 128533,
"=^_^=": 128568,
"=o": 128558,
"=p": 128539,
"=|": 128528,
">.<": 128547,
">:(": 128545,
">:(X": 128581,
">:-(": 128545,
">:D<": 129303,
">=(": 128545,
">_<": 128547,
"B-)": 128526,
"D:": 128550,
"O.O": 128562,
"O:)": 128519,
"O:-)": 128519,
"O=)": 128519,
T_T: 128546,
"V.v.V": 129408,
"X(": 128565,
"X-(": 128565,
"X-O": 128565,
"X-o": 128565,
"\\m/": 129304,
"\\o": 128587,
"^_^": 128513,
"^_^;;": 128517,
"o.o": 128558,
"o/": 128587,
"o_o;": 128531,
u_u: 128532,
x_x: 128565,
"}:)": 128520,
"}:-)": 128520,
"}=)": 128520,
"~@~": 128169
var Hoa = [[8419, 8419], [8505, 8505], [8596, 8601], [8617, 8618], [8986, 8987], [9E3, 9E3], [9167, 9167], [9193, 9203], [9208, 9210], [9410, 9410], [9642, 9643], [9654, 9654], [9664, 9664], [9723, 9726], [9728, 9732], [9742, 9742], [9745, 9745], [9748, 9749], [9752, 9752], [9757, 9757], [9760, 9760], [9762, 9763], [9766, 9766], [9770, 9770], [9774, 9775], [9784, 9786], [9792, 9792], [9794, 9794], [9800, 9811], [9824, 9824], [9827, 9827], [9829, 9830], [9832, 9832], [9851, 9851], [9855, 9855], [9874, 9879], [9881, 9881], [9883, 9884], [9888, 9889], [9898, 9899], [9904, 9905], [9917,
9918], [9924, 9925], [9928, 9928], [9934, 9935], [9937, 9937], [9939, 9940], [9961, 9962], [9968, 9973], [9975, 9978], [9981, 9981], [9986, 9986], [9989, 9989], [9992, 9997], [9999, 9999], [10002, 10002], [10004, 10004], [10006, 10006], [10013, 10013], [10017, 10017], [10024, 10024], [10035, 10036], [10052, 10052], [10055, 10055], [10060, 10060], [10062, 10062], [10067, 10069], [10071, 10071], [10083, 10084], [10133, 10135], [10145, 10145], [10160, 10160], [10175, 10175], [10548, 10549], [11013, 11015], [11035, 11036], [11088, 11088], [11093, 11093], [12336, 12336], [12349,
12349], [12951, 12951], [12953, 12953], [126980, 126980], [127183, 127183], [127344, 127345], [127358, 127359], [127374, 127374], [127377, 127386], [127462, 127487], [127489, 127490], [127514, 127514], [127535, 127535], [127538, 127546], [127568, 127569], [127744, 127777], [127780, 127891], [127894, 127895], [127897, 127899], [127902, 127984], [127987, 127989], [127991, 128253], [128255, 128317], [128329, 128334], [128336, 128359], [128367, 128368], [128371, 128378], [128391, 128391], [128394, 128397], [128400, 128400], [128405, 128406], [128420, 128421], [128424,
128424], [128433, 128434], [128444, 128444], [128450, 128452], [128465, 128467], [128476, 128478], [128481, 128481], [128483, 128483], [128488, 128488], [128495, 128495], [128499, 128499], [128506, 128591], [128640, 128709], [128715, 128722], [128736, 128741], [128745, 128745], [128747, 128748], [128752, 128752], [128755, 128758], [129296, 129310], [129312, 129319], [129328, 129328], [129331, 129338], [129340, 129342], [129344, 129349], [129351, 129355], [129360, 129374], [129408, 129425], [129472, 129472], [1041637, 1041646], [1042475, 1042476], [1042478,
1042487]],
Gmail
;-););^):-):-*:-/:-|:-D:-o:-P:-X:'(:(:(:):(|):):{:*:D:P[:|]}:-)=)=DB-)X-(>.<+/'\</3<3V.v.V~@~\m/JS Code
Since I couldn’t find an official and complete Google reference document, I looked at the current Javascript source code of Gmail/Hangouts.
ASCII Emoticons
-_--<@%;_;;-);-*;-p;-P;);*;p;P:-,:-(:-):-)X:-*:-/:-\:-D:-o:-O:-p:-P:-s:-S:,:''(:''D:'(:(:):(:):)X:*:/:\:3:C:D:o:O:p:P:s:S:X)!:-)!:)(]:{(=^..^=)(=^.^=)(n)(N)(y)(Y)}:-)}:)}=)\m/\o^_^;;^_^</3<\3<3='(=(=)=*=/=\=^_^==D=o=O=p=P>_<>:-(>:(>:(X>:D<>.<>=(~@~B-)D:o_o;O:-)O:)o.oO.Oo/O=)T_Tu_uV.v.Vx_xX-(X-oX-OX(https://mail.google.com/mail/e/{lowercase hexadecimal Unicode}Emoji Ranges
PUA
Flags
Key tops
Slash Commands
/algebraic/anjali/cling/disapprove/eightball,/8ball/facepalm/flowerbeam/happy/idk/lgtm/lit/octodisco/puppyparty/sadsauce/shame/shrug,shruggie/success,/successkid/tableback/tableflip/that/this/wizard,/wizardbeam/yuno,/whyyounoProposals
Conclusion
The only emoji missing for full Gmail support is a moustache: