Bonding gas piping, flexible CSST in conjuction with Iron pipe
The furnace already has bonded the gas system...just not well enough for CSST purposes
In your current setup, the gas piping (both CSST and black iron) is bonded to the electrical system via the equipment grounding conductor of the furnace.  This is typical practice for black-iron systems, but isn't good enough for CSST, which needs a much lower resistance path to the rest of the grounding and bonding system to keep excessive amounts of lightning energy from flowing on the CSST, creating arcs and pinholes as it goes.
In particular, you need to run a 6AWG copper wire from a grounding clamp on the black iron (preferably on the manifold, or on the black iron run to the furnace if that isn't possible) to some point on the grounding electrode conductor system that is within 75' of the bonding point. This is set out in IRC G2411.2.1 thru 2.3 (aka IFGC 310.2.1 thru 2.3):

G2411.2.1 (310.2.1)Point of connection.
The bonding jumper shall connect to a metallic pipe, pipe fitting, or CSST fitting.
G2411.2.2 (310.2.2)Size and material of jumper.
The bonding jumper shall not be smaller than 6AWG copper wire or equivalent.
G2411.2.3 (310.2.3)Bonding jumper length.
The length of the bonding jumper between the connection to a gas piping system and the connection to a grounding electrode system shall not exceed 75 feet (22 860mm).  Any additional grounding electrodes installed to meet this requirement shall be bonded to the electrical service grounding electrode system, or where installed, the lightning protection grounding electrode system.

Parts-wise, you can use any ordinary pipe grounding clamp to attach the wire to the pipe, and a split-bolt or lay-in T-tap (Ilsco GTT-2-2 or equivalent) will work for attaching the bonding wire to the grounding electrode conductor.