Your code does work in the Gremlin console::

gremlin> graph = JanusGraphFactory.open('conf/janusgraph-inmemory.properties')
==>standardjanusgraph[inmemory:[127.0.0.1]]
gremlin> g = graph.traversal()
==>graphtraversalsource[standardjanusgraph[inmemory:[127.0.0.1]], standard]
gremlin> shape = Geoshape.geoshape(Geoshape.getShapeFactory().polygon().pointXY(42.5636137, 1.4294247).pointXY(42.5634345, 1.4286752).pointXY(42.5631842,
......1> 1.4276233).pointXY(42.5631815, 1.4276138).pointXY(42.5630055, 1.4269872).pointXY(42.562931, 1.4265631).pointXY(42.5629338, 1.4261425).pointXY(42.5630545, 1.4257756).pointXY(42.5631548,
......
.....50> 1.4276864).pointXY(42.5666263, 1.4292844).pointXY(42.5649079, 1.4304155).pointXY(42.5636137, 1.4294247).build())
==>POLYGON ((42.5636137 1.4294247, 42.5634345 1.4286752, 42.5631842 1.4276233, 42.5631815 1.4276138, 42.5630055 1.4269872, 42.562931 1.4265631, 42.5629338 1.4261425, 42.5630545 1.4257756, 42.5631548 1.4255639, 42.5632525 1.4253578,
.....
42.5673518 1.4176981, 42.5674387 1.4181612, 42.5674788 1.4188149, 42.5674253 1.4202223, 42.5671779 1.4215116, 42.5675657 1.4222743, 42.567198 1.4234637, 42.5668502 1.4248438, 42.5668034 1.4254431, 42.5665996 1.4256616, 42.5663789 1.4256616, 42.5662986 1.4257615, 42.5662385 1.4260521, 42.5662585 1.4270327, 42.5663254 1.4276864, 42.5666263 1.4292844, 42.5649079 1.4304155, 42.5636137 1.4294247))
gremlin> g.addV("place").property("location", shape).iterate()
gremlin> g.V().elementMap()
07:56:52 WARN  org.janusgraph.graphdb.transaction.StandardJanusGraphTx  - Query requires iterating over all vertices [()]. For better performance, use indexes
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gremlin>

What REPL did you use? Gremlin Console? Python?

Best wishes,     Marc