Filing a Flight plan in IFR style (with airways etc.) is preferred by ATC but not mandatory. Still, even without airways, using IFR fixes (point/navaid to point/navaid) style is preferred. VFR flights are preferred to fly a track below or within lateral limits of controlled airspace (at +500ft. ALT). VFR via non controlled airspace is not allowed when above Greek territory. Greek territory is considered land areas and any sea surface 12nm inside from any Greek shore. As stated in Greece AIP, in the "high seas" i.e. above sea, outside the 12nm from any shore, there is no such restriction.
Airways, TMA's & CTR's are controlled airspace though and most of Greek territory - and 12nm sea from shore - is covered by the lateral limits of one of them so there is no problem with this rule. Inside some of the TMA's there are Greece AIP published suggested "VFR routes" with specific altitudes that can be flown without the need of entry clearance. Diversions from filed route & altitude are given if requested (coordination with FIS) en route for e.g. "weather/wind turbulence avoidance" but not during the initial Flight Plan submission phase on ground. All VFR routes charts are available at airports where CAA office has the Greece AIP. Military MTMAs covering most of mainland Greece do have some suggested VFR routes that are published in the Military AIP (MAIP) which is not publicly available. Military ATC on a busy day MAY ask you to follow them even though not published on civil charts. Only reported problems have been reported at areas around Zakynthos LGZA where Andravida LGAD MTMA MIL ATC handle a lot of civil traffic in the summer for Zakynthos LGZA and their own military traffic for Andravida LGAD and Araxos LGRX air force bases.
These routes ARE charted in the Greek Aviation Maps (GAM) documented in paragraph 20.
So in case you want to fly low level VFR in areas:
A) East of Zakynthos LGZA (inside Andravida LGAD MTMA)
B) Northwest of Athens (Tanagra LGTG MTMA)
C) or in central east mainland Greece (LGBL and LGLR MTMAs)
these charts may come handy.
In all other civil TMAs of Greece VFR routes are published in the civil airport charts (AIP charts, GAM charts and Jeppesen VFR Bottlang guides).