Typically, hollow-core fiber (HCF)-based laser gas sensing systems use a single monolithic fiber as a gas cell. This results in a tradeoff between sensitivity which requires long optical fiber and sensor’s response time, that grows with the fiber length. Here we present a simple approach to solve this issue with new all-fiber modular gas cell design. The setup uses modified fiber mating sleeves to connect multiple HCFs in series, with optical loss of typically from 2 to 3 dB per connection. By injecting the sample gas at every second HCF-to-HCF connection, all HCF segments are filled simultaneously. We demonstrated the setups with two and four HCF segments, each with a length of 1.35 m. Laser absorption spectroscopy of methane near 1687 nm is used for setup characterization. Using the inlet pressure of 2 bar, the gas filling time is reduced over 13 times, from 77.9 s for a monolithic fiber, to only 5.9 s for the four-segment design (total HCF length of 5.4 m). With the pressure increased to 3 bar, the response time is further decreased to 3.6 s. Pressure buildup and drawdown times in the HCF were also examined.