コマンド
\usepackage[english,greek]{babel}
MikTeX 2.9 を永久に実行します。
OS Windows 7
出力は
This is pdfTeX, Version 3.1415926-2.4-1.40.13 (MiKTeX 2.9)
entering extended mode
(C:/Users/x/AppData/Local/Temp/lyx_tmpdir.Hp3100/lyx_tmpbuf2/negreponti-1.tex
LaTeX2e <2011/06/27>
Babel <v3.8m> and hyphenation patterns for english, afrikaans, ancientgreek, ar
abic, armenian, assamese, basque, bengali, bokmal, bulgarian, catalan, coptic,
croatian, czech, danish, dutch, esperanto, estonian, farsi, finnish, french, ga
lician, german, greek, gujarati, hindi, hungarian, icelandic, indonesian, inter
lingua, irish, italian, kannada, kurmanji, latin, latvian, lithuanian, malayala
m, marathi, mongolian, mongolianlmc, monogreek, ngerman, nynorsk, oriya, panjab
i, pinyin, polish, portuguese, romanian, russian, sanskrit, serbian, slovak, sl
ovenian, spanish, swedish, swissgerman, tamil, telugu, turkish, turkmen, ukengl
ish, ukrainian, uppersorbian, usenglishmax, welsh, loaded.
編集済み:OP のコメントからの小さなコード:
\makeatletter \makeatother
\documentclass[11pt,oneside]{amsart}
\usepackage[iso-8859-7]{inputenc}
\usepackage[a4paper]{geometry}
\usepackage{amsthm}
\usepackage{amstext}
\usepackage{amssymb}
\makeatletter
\usepackage[english,greek]{babel}
%\usepackage[default]{gfsneohellenic}
%\usepackage[LGR,T1]{fontenc} %% LGR encoding is needed for gfsneohellenic
\makeatother
\begin{document} klkk \greektext γφγγ
\end{document}