Use MODTCL_WISH_ADJ; drop a patch.
MODTK_VERSION=8.6


diff -Nurp /usr/ports/geo/gpsbabel/Makefile ./Makefile
--- /usr/ports/geo/gpsbabel/Makefile	Fri Jul 25 08:48:26 2025
+++ ./Makefile	Fri Nov 21 08:40:18 2025
@@ -14,6 +14,9 @@ DISTNAME=		gpsbabel-${VERSION}
 PKGNAME-main=		gpsbabel-${VERSION}
 PKGNAME-tk=		gpsbabel-tk-${VERSION}
 PKGNAME-qt=		gpsbabel-qt-${VERSION}
+
+REVISION-tk=		0
+
 CATEGORIES=		geo
 
 HOMEPAGE=		https://www.gpsbabel.org/
@@ -37,6 +40,8 @@ MODULES=		devel/cmake \
 			x11/tk \
 			x11/qt6
 
+MODTK_VERSION=		8.6
+
 # gpsbabel (from the -main subpackage) is linked against Qt6Core
 LIB_DEPENDS-main=	devel/libusb1 \
 			devel/shapelib \
@@ -60,7 +65,7 @@ CXXFLAGS +=		-I${LOCALBASE}/include
 MODCMAKE_LDFLAGS=	-L${LOCALBASE}/lib -lshp
 
 pre-configure:
-	${SUBST_CMD} ${WRKSRC}/guibabel
+	@${MODTCL_WISH_ADJ} ${WRKSRC}/guibabel
 	@perl -pi -e 's/gpsbabelfe-bin/guibabel-qt/' \
 		${WRKSRC}/gui/gpsbabel.desktop
 
diff -Nurp /usr/ports/geo/gpsbabel/patches/patch-guibabel ./patches/patch-guibabel
--- /usr/ports/geo/gpsbabel/patches/patch-guibabel	Fri Mar 11 14:06:12 2022
+++ ./patches/patch-guibabel	Wed Dec 31 19:00:00 1969
@@ -1,10 +0,0 @@
---- guibabel.orig	Thu Oct 12 19:37:40 2006
-+++ guibabel	Thu Oct 12 19:37:51 2006
-@@ -1,6 +1,6 @@
- #!/bin/sh
- # the next line restarts using wish \
--exec wish "$0" "$@"
-+exec wish${MODTK_VERSION} "$0" "$@"
- 
- # 
- # This was largely cribbed from the TCL demo code.  My TCL skills weren't