Context. Sub-Neptune exoplanets such as GJ 1214 b provide a critical link between terrestrial and giant planets, yet atmospheric characterisation remains challenging due to high-altitude clouds and compressed atmospheres. JWST has recently hinted at molecular signals in GJ 1214 b, and ground-based high-resolution spectroscopy is potentially able to confirm them. Aims. We aim to constrain the atmospheric composition of GJ 1214 b using all available transits observed with the upgraded CRIRES+ spectrograph on the Very Large Telescope (VLT) by searching for the signatures of water vapour, methane, and carbon dioxide. Methods. We analysed eight CRIRES+ transit datasets covering the K band (1.90-2.45 μm) at a resolving power of R ≍ 100,000. We used the SysRem algorithm to correct for telluric and stellar contributions and employed the cross-correlation technique with templates from petitRADTRANS to search for H2O, CH4, and CO2. Injection-recovery tests were performed across a grid of metallicities (Z) and cloud-deck pressures (pc) to quantify detection limits. We also generated predictions for ANDES observations using end-to-end simulated datasets with EXoPLORE. Results. We detect no significant H2O, CH4, or CO2 signatures. Injection-recovery tests show that such non-detections exclude atmospheres with low-altitude clouds and moderate or low metallicities. CH4 yields the tightest empirical limits, with CO2 unexpectedly ruling out intermediate metallicities (∼100× solar) with clouds deeper due to its rapidly rising opacity in compressed high-Z atmospheres. Our constraints are in line with either a high-Z or a high-altitude aerosol layer, in agreement with recent JWST inferences. Conclusions. The combined analysis of eight CRIRES+ datasets provides the most stringent high-resolution constraints on the atmospheric properties of GJ 1214 b to date. Planetary signals are likely buried below our current detection threshold, preventing confirmation of recent JWST-reported molecular hints. Simulations of a single transit observed with ANDES on the ELT predict modest improvements for H2O, a substantially expanded detectable region for CH4, and the strongest gains for CO2, making the latter a particularly effective tracer for characterising high-metallicity atmospheres in sub-Neptunes.