core/txpool: fix minor flaw in isGapped check (ethereum#27404)

This fix a minor implementation issue with the newly introduced isGapped function and it is described in ticket: ethereum#27401

core/txpool/txpool.go

@@ -823,10 +823,12 @@ func (pool *TxPool) add(tx *types.Transaction, local bool) (replaced bool, err e
 
 // isGapped reports whether the given transaction is immediately executable.
 func (pool *TxPool) isGapped(from common.Address, tx *types.Transaction) bool {
-	// Short circuit if transaction matches pending nonce and can be promoted
-	// to pending list as an executable transaction.
+	// Short circuit if transaction falls within the scope of the pending list
+	// or matches the next pending nonce which can be promoted as an executable
+	// transaction afterwards. Note, the tx staleness is already checked in
+	// 'validateTx' function previously.
 	next := pool.pendingNonces.get(from)
-	if tx.Nonce() == next {
+	if tx.Nonce() <= next {
 		return false
 	}
 	// The transaction has a nonce gap with pending list, it's only considered