I tried it with SPOT by ./ltl2tgba -f "F (x>1)" and SPOT fails to parse it.

There are two ways to parse it:

Either quote the part you want Spot to consider as an atomic proposition:

ltl2tgba 'F("x > 1")'

Or use --lenient to allow any parenthesized block that Spot do not understand to be considered as an atomic proposition

ltl2tgba --lenient 'F(x > 1)'

See this page for more examples: http://spot.lip6.fr/userdoc/ioltl.html