You will have to use the eval feature, which has been added to GAMS in distribution 147. I also include an example to compute N+1 from N:

$ontext
If you want to do something like
 $setglobal N 10
 set i /1*(%N%+1)/
here's how.
$offtext

* force a compilation error if we don't run GAMS Rev 147 or higher
$version 147

* if we get here we're new enough to use the $eval feature
$setglobal N 10
$eval NP1 %N%+1
set I / 1 * %NP1% /;
display I;

Finally, some details about what GAMS systems will support: the $eval call is mentioned in our release notes for distribution 22.6 (GAMS Base Rev 149), but it is actually available in some form in Distribution 22.4 (GAMS Base Rev 147).