You can do:

v f(%`

f( moves the the first parenthesis

% moves to the matching parenthesis

Another way if your are at b and to follow your idea is:

va(obw

va( select (gunc(a, b))

o switch the cursor in front od the selection

bw extend it to func(gunc(a, b))

You can do:

vf(%`

f( moves the the first parenthesis

% moves to the matching parenthesis

Another way if your are at b and to follow your idea is:

va(ob

va( select (gunc(a, b))

o switch the cursor in front od the selection

b extend it to func(gunc(a, b))

You can do:

v f(%`

f( moves the the first parenthesis

% moves to the matching parenthesis

You can do:

v f(%`

f( moves the the first parenthesis

% moves to the matching parenthesis

Another way if your are at b and to follow your idea is:

va(obw

va( select (gunc(a, b))

o switch the cursor in front od the selection

bw extend it to func(gunc(a, b))