This is a feature request.
Today, when running emacs server and no frames already exists,
emacsclient creates a terminal frame even when a window system is
available. The only way to make emacsclient create GUI frames is by
using the "-c" option which *always* creates a new frame, but negates
the desirable behavior of reusing existing frames.

I'd like to propose a new emacsclient option or an emacs
customizable variable so that the following pseudo-code happens when
invoking "emacsclient foo.txt" (or "emacsclient --new-option foo.txt"):

if (exist-frame)
     open-buffer-in-existing-frame
else if (window-system-available)
     open-buffer-in-new-GUI-frame
else
     open-buffer-in-new-terminal-frame

There was also some discussion about this on r/emacs
(
https://www.reddit.com/r/emacs/comments/8iq4ho/how_make_emacsclient_create_gui_frame_by_default/
)
and I was directed at making this feature request.

In GNU Emacs 25.2.2 (x86_64-pc-linux-gnu, GTK+ Version 3.22.21)
 of 2017-09-22, modified by Debian built on lgw01-amd64-050
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description:    Ubuntu 18.04 LTS

Configured using:
 'configure --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --with-pop=yes
 --enable-locallisppath=/etc/emacs25:/etc/emacs:/usr/local/share/emacs/25.2/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/25.2/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --build x86_64-linux-gnu
 --prefix=/usr --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --with-pop=yes
 --enable-locallisppath=/etc/emacs25:/etc/emacs:/usr/local/share/emacs/25.2/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/25.2/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-x=yes --with-x-toolkit=gtk3
 --with-toolkit-scroll-bars 'CFLAGS=-g -O2
 -fdebug-prefix-map=/build/emacs25-jYekUr/emacs25-25.2+1=.
-fstack-protector-strong
 -Wformat -Werror=format-security -Wall' 'CPPFLAGS=-Wdate-time
 -D_FORTIFY_SOURCE=2' 'LDFLAGS=-Wl,-Bsymbolic-functions -Wl,-z,relro''

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY
ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11

Important settings:
  value of $LC_CTYPE: pt_BR.UTF-8
  value of $LC_MONETARY: pt_BR.UTF-8
  value of $LC_NUMERIC: pt_BR.UTF-8
  value of $LC_TIME: pt_BR.UTF-8
  value of $LANG: pt_BR.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Fundamental

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
Loading /etc/emacs/site-start.d/50dictionaries-common.el (source)...
Loading debian-ispell...
Loading /var/cache/dictionaries-common/emacsen-ispell-default.el
(source)...done
Loading debian-ispell...done
Loading /var/cache/dictionaries-common/emacsen-ispell-dicts.el
(source)...done
Loading /etc/emacs/site-start.d/50dictionaries-common.el (source)...done
Loading /etc/emacs/site-start.d/50python-docutils.el (source)...done
For information about GNU Emacs and the GNU system, type C-h C-a.
Making completion list...
user-error: End of history; no default available [2 times]

Load-path shadows:
/usr/share/emacs/25.2/site-lisp/debian-startup hides
/usr/share/emacs/site-lisp/debian-startup
/usr/share/emacs/site-lisp/rst hides
/usr/share/emacs/25.2/lisp/textmodes/rst

Features:
(shadow sort mail-extr emacsbug message dired format-spec rfc822 mml
mml-sec password-cache epg epg-config gnus-util mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rfc2047 rfc2045 ietf-drums mm-util help-fns help-mode easymenu
cl-loaddefs pcase cl-lib mail-prsvr mail-utils thingatpt time-date
mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel x-win term/common-win x-dnd tool-bar dnd fontset
image regexp-opt fringe tabulated-list newcomment elisp-mode lisp-mode
prog-mode register page menu-bar rfn-eshadow timer select scroll-bar
mouse jit-lock font-lock syntax facemenu font-core frame cl-generic cham
georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao
korean japanese eucjp-ms cp51932 hebrew greek romanian slovak czech
european ethiopic indian cyrillic chinese charscript case-table epa-hook
jka-cmpr-hook help simple abbrev minibuffer cl-preloaded nadvice
loaddefs button faces cus-face macroexp files text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote dbusbind inotify dynamic-setting
system-font-setting font-render-setting move-toolbar gtk x-toolkit x
multi-tty make-network-process emacs)

Memory information:
((conses 16 89661 10198)
 (symbols 48 19887 0)
 (miscs 40 94 196)
 (strings 32 14841 5123)
 (string-bytes 1 422977)
 (vectors 16 11895)
 (vector-slots 8 431894 5412)
 (floats 8 165 56)
 (intervals 56 300 0)
 (buffers 976 20))